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>

drivers/gpu/drm/amd/display/modules/color/color_gamma.c

@@ -1854,6 +1854,8 @@ bool mod_color_calculate_degamma_params(struct dc_transfer_func *input_tf,
 			coordinates_x, axis_x, curve,
 			MAX_HW_POINTS, tf_pts,
 			mapUserRamp && ramp && ramp->type == GAMMA_RGB_256);
+	if (ramp->type == GAMMA_CUSTOM)
+		apply_lut_1d(ramp, MAX_HW_POINTS, tf_pts);
 
 	ret = true;