drm/amd/display: handle empty LUTs in __set_input_tf

Unlike degamma, blend gamma doesn't support hardcoded curve
(predefined/ROM), but we can use AMD color module to fill blend gamma
parameters when we have non-linear plane gamma TF without plane gamma
LUT. The regular degamma path doesn't hit this.
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Joshua Ashton <joshua@froggi.es>
Signed-off-by: Melissa Wen <mwen@igalia.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_color.c

@@ -578,6 +578,7 @@ static int __set_input_tf(struct dc_transfer_func *func,
 	struct dc_gamma *gamma = NULL;
 	bool res;
 
+	if (lut_size) {
 		gamma = dc_create_gamma();
 		if (!gamma)
 			return -ENOMEM;
@@ -586,8 +587,11 @@ static int __set_input_tf(struct dc_transfer_func *func,
 		gamma->num_entries = lut_size;
 
 		__drm_lut_to_dc_gamma(lut, gamma, false);
+	}
 
-	res = mod_color_calculate_degamma_params(NULL, func, gamma, true);
+	res = mod_color_calculate_degamma_params(NULL, func, gamma, gamma != NULL);
+
+	if (gamma)
 		dc_gamma_release(&gamma);
 
 	return res ? 0 : -ENOMEM;