drm/i915/mcr: Hold GT forcewake during steering operations

The steering control and semaphore registers are inside an "always on"
power domain with respect to RC6.  However there are some issues if
higher-level platform sleep states are entering/exiting at the same time
these registers are accessed.  Grabbing GT forcewake and holding it over
the entire lock/steer/unlock cycle ensures that those sleep states have
been fully exited before we access these registers.

This is expected to become a formally documented/numbered workaround
soon.

Note that this patch alone isn't expected to have an immediately
noticeable impact on MCR (mis)behavior; an upcoming pcode firmware
update will also be necessary to provide the other half of this
workaround.

v2:
 - Move the forcewake inside the Xe_LPG-specific IP version check.  This
   should only be necessary on platforms that have a steering semaphore.

Fixes: 3100240bf846 ("drm/i915/mtl: Add hardware-level lock for steering")
Cc: Radhakrishna Sripada <radhakrishna.sripada@intel.com>
Cc: Jonathan Cavitt <jonathan.cavitt@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Radhakrishna Sripada <radhakrishna.sripada@intel.com>
Reviewed-by: Jonathan Cavitt <jonathan.cavitt@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231019170241.2102037-2-matthew.d.roper@intel.com

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