drm/amd/display: Fix incorrectly pruned modes with deep color

[Why]
When "max bpc" is set to enable deep color, some modes are removed from
the list if they fail validation on max bpc. These modes should be kept
if they validates fine with lower bpc.

[How]
- Retry with lower bpc in mode validation.
- Same in atomic commit to apply working bpc, not necessarily max bpc.

Signed-off-by: Stylon Wang <stylon.wang@amd.com>
Reviewed-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>