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: d2dea1f ("drm/amd/display: Generalize new minimal transition path")
Reviewed-by: Alvin Lee <[email protected]>
Acked-by: Tom Chung <[email protected]>
Signed-off-by: Wenjing Liu <[email protected]>
Tested-by: Daniel Wheeler <[email protected]>
Signed-off-by: Alex Deucher <[email protected]>

Author: Wenjing Liu

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

@@ -3024,7 +3024,8 @@ static void backup_planes_and_stream_state(
 		scratch->blend_tf[i] = *status->plane_states[i]->blend_tf;
 	}
 	scratch->stream_state = *stream;
-	scratch->out_transfer_func = *stream->out_transfer_func;
+	if (stream->out_transfer_func)
+		scratch->out_transfer_func = *stream->out_transfer_func;
 }
 
 static void restore_planes_and_stream_state(
@@ -3046,7 +3047,8 @@ static void restore_planes_and_stream_state(
 		*status->plane_states[i]->blend_tf = scratch->blend_tf[i];
 	}
 	*stream = scratch->stream_state;
-	*stream->out_transfer_func = scratch->out_transfer_func;
+	if (stream->out_transfer_func)
+		*stream->out_transfer_func = scratch->out_transfer_func;
 }
 
 static bool update_planes_and_stream_state(struct dc *dc,