drm/amd/display: Re-add custom degamma support

[Why]
The dc_gamma_type CUSTOM_GAMMA is used to represent degamma
mappings passed in by drm. This type of gamma must be interpolated
into a transfer function by apply_1d_lut.  The line in
mod_color_calculate_degamma_params that handled this case
was erroneously removed.

[How]
For CUSTOM_GAMMA degamma, calculate the lut as before.

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>