drm/amd/display: Check ramp != NULL before applying lut1d for degamma

[Why]
A NULL ramp is a valid configuration for passing into
mod_color_calculate_degamma_params but we'll hit a NULL pointer if we do
so.

We need this in order to get the right transfer function to do degamma
on NV12 formats where we aren't supplied with a custom user degamma.

[How]
Add the NULL check.

Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Reviewed-by: Zhan Liu <Zhan.Liu@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>