drm/amd/display: Only use depth 36 bpp linebuffers on DCN display engines.

drm/amd/display: Only use depth 36 bpp linebuffers on DCN display engines.

Various DCE versions had trouble with 36 bpp lb depth, requiring fixes,
last time in commit 8c984886fc67 ("drm/amd/display: Fix 10bit 4K display
on CIK GPUs") for DCE-8. So far >= DCE-11.2 was considered ok, but now I
found out that on DCE-11.2 it causes dithering when there shouldn't be
any, so identity pixel passthrough with identity gamma LUTs doesn't work
when it should. This breaks various important neuroscience applications,
as reported to me by scientific users of Polaris cards under Ubuntu 22.04
with Linux 5.15, and confirmed by testing it myself on DCE-11.2.

Lets only use depth 36 for DCN engines, where my testing showed that it
is both necessary for high color precision output, e.g., RGBA16 fb's,
and not harmful, as far as more than one year in real-world use showed.

DCE engines seem to work fine for high precision output at 30 bpp, so
this ("famous last words") depth 30 should hopefully fix all known problems
without introducing new ones.

Successfully retested on DCE-11.2 Polaris and DCN-1.0 Raven Ridge on
top of Linux 5.19.0-rc2 + drm-next.

Fixes: 8c984886fc67 ("drm/amd/display: Fix 10bit 4K display on CIK GPUs")
Signed-off-by: Mario Kleiner <mario.kleiner.de@gmail.com>
Tested-by: Mario Kleiner <mario.kleiner.de@gmail.com>
Cc: stable@vger.kernel.org # 5.14.0
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>