drm/radeon: Track the status of a page flip more explicitly

This prevents a panic: radeon_crtc_handle_page_flip() could run before
radeon_flip_work_func(), triggering the BUG_ON() in drm_vblank_put().

Tested-by: Dieter Nützel <Dieter@nuetzel-hh.de>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
diff --git a/drivers/gpu/drm/radeon/radeon_mode.h b/drivers/gpu/drm/radeon/radeon_mode.h
index 98a125d..0592ddb 100644
--- a/drivers/gpu/drm/radeon/radeon_mode.h
+++ b/drivers/gpu/drm/radeon/radeon_mode.h
@@ -306,6 +306,12 @@
 	uint16_t amount;
 };
 
+enum radeon_flip_status {
+	RADEON_FLIP_NONE,
+	RADEON_FLIP_PENDING,
+	RADEON_FLIP_SUBMITTED
+};
+
 struct radeon_crtc {
 	struct drm_crtc base;
 	int crtc_id;
@@ -331,6 +337,7 @@
 	/* page flipping */
 	struct workqueue_struct *flip_queue;
 	struct radeon_flip_work *flip_work;
+	enum radeon_flip_status flip_status;
 	/* pll sharing */
 	struct radeon_atom_ss ss;
 	bool ss_enabled;