drm/amd/display: Revert adding degamma coefficients

[Why]
Degamma coefficients are calculated in our degamma formula using
the regamma coefficients. We do not need to add separate degamma
coefficients.

[How]
Remove the change to add separate degamma coefficients.

Reviewed-by: Krunoslav Kovac <Krunoslav.Kovac@amd.com>
Acked-by: Mikita Lipski <mikita.lipski@amd.com>
Signed-off-by: Jaehyun Chung <jaehyun.chung@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>