drm/amd/display: Only enable common modes for eDP and LVDS

[Why]
The main reason common modes are added is for compatibility with
clone mode when a laptop is connected to a projector or external
monitor.  Since commit 978fa2f6d0b12 ("drm/amd/display: Use scaling
for non-native resolutions on eDP") when non-native modes are picked
for eDP the GPU scalar will be used.  This is because it is inconsistent
whether eDP panels have the capability to actually drive non-native
resolutions. With panels connected to other connectors this limitation
generally doesn't exist as we the EDID will advertise support for a
number of resolutions and monitors will use built in scaling hardware.

Comparing DC and non-DC code paths the non-DC code path only adds
common modes for LVDS and eDP whereas the DC codepath does it for
all connector types.

In the past there was an experiment done to disable common mode adding
for eDP and LVDS from commit 6d396e7ac1ce3 ("drm/amd/display: Disable
common modes for LVDS") and commit 7948afb46af92 ("drm/amd/display:
Disable common modes for eDP") but this was reverted in
commit a8b79b09185de ("drm/amd: Re-enable common modes for eDP and
LVDS") because it caused problems with Xorg.

[How]
Only add common modes for eDP and LVDS for DC, matching the behavior
of non-DC.

Suggested-by: Timur Kristóf <timur.kristof@gmail.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Reviewed-by: Timur Kristóf <timur.kristof@gmail.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Link: https://lore.kernel.org/r/20250924161624.1975819-2-mario.limonciello@amd.com
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

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