drm/amd/display: restore code for plane with no modifiers

When this file was split in commit 5d945cbc ("drm/amd/display:
Create a file dedicated to planes") this chunk seemed to get dropped.

Linus noticed on this rx580 and I've reproduced on FIJI which makes
sense as these are pre-modifier GPUs.

With this applied, I get gdm back.

Fixes: 5d945cbc ("drm/amd/display: Create a file dedicated to planes")
Signed-off-by: Dave Airlie <airlied@redhat.com>
Reported-and-tested-by: Linus Torvalds <torvalds@linux-foundation.org>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

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

@@ -1591,6 +1591,9 @@ int amdgpu_dm_plane_init(struct amdgpu_display_manager *dm,
 	if (res)
 		return res;
 
+	if (modifiers == NULL)
+		adev_to_drm(dm->adev)->mode_config.fb_modifiers_not_supported = true;
+
 	res = drm_universal_plane_init(adev_to_drm(dm->adev), plane, possible_crtcs,
 				       &dm_plane_funcs, formats, num_formats,
 				       modifiers, plane->type, NULL);