drm/amd/display: fix a dereference of a NULL pointer

[why&how]
In some platform out_transfer_func may not be popualted. We need to check
for null before dereferencing it.

Fixes: d2dea1f1 ("drm/amd/display: Generalize new minimal transition path")
Reviewed-by: Alvin Lee <alvin.lee2@amd.com>
Acked-by: Tom Chung <chiahsuan.chung@amd.com>
Signed-off-by: Wenjing Liu <wenjing.liu@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dc/core/dc.c

@@ -3057,6 +3057,7 @@ static void backup_planes_and_stream_state(
 		scratch->blend_tf[i] = *status->plane_states[i]->blend_tf;
 	}
 	scratch->stream_state = *stream;
+	if (stream->out_transfer_func)
 		scratch->out_transfer_func = *stream->out_transfer_func;
 }
 
@@ -3079,6 +3080,7 @@ static void restore_planes_and_stream_state(
 		*status->plane_states[i]->blend_tf = scratch->blend_tf[i];
 	}
 	*stream = scratch->stream_state;
+	if (stream->out_transfer_func)
 		*stream->out_transfer_func = scratch->out_transfer_func;
 }