drm/i915/cmtg: Disable the CMTG

The CMTG is a timing generator that runs in parallel with transcoders
timing generators and can be used as a reference for synchronization.

We have observed that we are inheriting from GOP a display configuration
with the CMTG enabled. Because our driver doesn't currently implement
any CMTG sequences, the CMTG ends up still enabled after our driver
takes over.

We need to make sure that the CMTG is not enabled if we are not going to
use it. For that, let's add a partial implementation in our driver that
only cares about disabling the CMTG if it was found enabled during
initial hardware readout. In the future, we can also implement sequences
for using the CMTG if that becomes a needed feature.

For now, we only deal with cases when it is possible to disable the CMTG
without requiring a modeset. For earlier display versions, we simply
skip if we find the CMTG enabled and we can't disable it without a
proper modeset. In the future, we need to properly handle that case.

v2:
 - DG2 does not have the CMTG. Update HAS_CMTG() accordingly.
 - Update logic to force disabling of CMTG only for initial commit.
v3:
 - Add missing changes for v2 that were staged but not committed.
v4:
 - Avoid if/else duplication in intel_cmtg_dump_state() by using "n/a"
   for CMTG B enabled/disabled string for platforms without it. (Jani)
 - Prefer intel_cmtg_readout_hw_state() over intel_cmtg_readout_state().
   (Jani)
 - Use display struct instead of i915 as first parameter for
   TRANS_DDI_FUNC_CTL2(). (Jani)
 - Fewer continuation lines in variable declaration/initialization for
   better readability. (Jani)
 - Coding style improvements. (Jani)
 - Use drm_dbg_kms() instead of drm_info() for logging the disabling
   of the CMTG.
 - Make struct intel_cmtg_state entirely private to intel_cmtg.c.
v5:
 - Do the disable sequence as part of the sanitization step after
   hardware readout instead of initial modeset commit. (Jani)
 - Adapt to commit 15133582465f ("drm/i915/display: convert global state
   to struct intel_display") by using a display struct instead of i915
   as argument for intel_atomic_global_obj_init().
v6:
 - Do not track CMTG state as a global state. (Ville)
 - Simplify the driver logic by only disabling the CMTG only on cases
   when a modeset is not required. (Ville)
v7:
 - Remove the call to drm_WARN_ON() when checking
   intel_cmtg_disable_requires_modeset() and use a FIXME in the comment
   instead.
 - Remove the !HAS_CMTG() guard from intel_cmtg_get_config(), which is
   static and its caller is already protected by that same condition.
 - Also take the opportunity to put some Bspec references in the commit
   trailers section.
v8:
 - Use HAS_TRANSCODER() instead of intel_crtc_for_pipe(). (Ville)
 - Ensure transcoder power well is enabled before reading
   TRANS_DDI_FUNC_CTL2. (Ville)

Bspec: 68915, 49262
Cc: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Gustavo Sousa <gustavo.sousa@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250124173956.46534-1-gustavo.sousa@intel.com

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