drm/amd/display: Implement custom degamma lut on dcn

[Why]
Custom degamma lut functions are a feature we would
like to support on compatible hardware

[How]
In atomic check, convert from array of drm_color_lut to
dc_transfer_func.  On hardware commit, allow for possibility
of custom degamma.  Both are based on the equivalent
regamma pipeline.

Signed-off-by: David Francis <David.Francis@amd.com>
Reviewed-by: Krunoslav Kovac <Krunoslav.Kovac@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>