drm/amd/display: fix crash with modesetting driver

They might call commit with ACTION_NOTHING without a mode.
We shouldn't crash but simply skip updating stream scaling
settings since scaling obviously didn't change without a
provided mode.
Signed-off-by: Harry Wentland <harry.wentland@amd.com>
Reviewed-by: Andrey Grodzovsky <Andrey.Grodzovsky@amd.com>
Acked-by: Harry Wentland <Harry.Wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

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

@@ -638,6 +638,10 @@ static void update_stream_scaling_settings(
 	struct rect src = { 0 }; /* viewport in composition space*/
 	struct rect dst = { 0 }; /* stream addressable area */
 
+	/* no mode. nothing to be done */
+	if (!mode)
+		return;
+
 	/* Full screen scaling by default */
 	src.width = mode->hdisplay;
 	src.height = mode->vdisplay;