drm/amd/display: add support for low bpc

[WHY&HOW]
Low bpc timings are failing validation, port a patch to allow them to pass.

Signed-off-by: Dillon Varone <dillon.varone@amd.com>
Acked-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>