drm/amd/display: DGAM enabled for HDR

[Why]
On HW that doesn't have input LUT, we may combine degamma with OS ramp
Problem here is that it assumes DGAM is inverse of PQ or SRGB. It doesn't
handle linear case, it would default to sRGB and always enable DGAM..

[How] Add handling for linear case. Also check for null ramp and instead of
blowing up, assume it's identity.

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