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: NNicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Reviewed-by: NZhan Liu <Zhan.Liu@amd.com>
Acked-by: NRodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>

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

@@ -1902,7 +1902,7 @@ bool mod_color_calculate_degamma_params(struct dc_transfer_func *input_tf,
 
 
 
-	if (ramp->type == GAMMA_CUSTOM)
+	if (ramp && ramp->type == GAMMA_CUSTOM)
 		apply_lut_1d(ramp, MAX_HW_POINTS, tf_pts);
 
 	ret = true;