drm/amd/display: add dmcub check on RENOIR

RENOIR loads dmub fw not dmcu, check dmcu only will prevent loading iram,
it breaks backlight control.

Bug: https://bugzilla.kernel.org/show_bug.cgi?id=208277
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Signed-off-by: Aaron Ma <aaron.ma@canonical.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org

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

@@ -1358,7 +1358,7 @@ static int dm_late_init(void *handle)
 	struct dmcu *dmcu = NULL;
 	bool ret;
 
-	if (!adev->dm.fw_dmcu)
+	if (!adev->dm.fw_dmcu && !adev->dm.dmub_fw)
 		return detect_mst_link_for_all_connectors(adev->ddev);
 
 	dmcu = adev->dm.dc->res_pool->dmcu;