drm/atomic: Only destroy connector states with connection mutex held

Otherwise the connector might have been unplugged and destroyed while
we didn't look. Yet another fallout from DP MST hotplugging that I
didn't consider.

To make sure we get this right add an appropriate WARN_ON to
drm_atomic_state_clear (obviously only when we actually have a state
to clear up). And reorder all the state_clear and backoff calls to
make it work out properly.

Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Reviewed-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
diff --git a/drivers/gpu/drm/drm_atomic_helper.c b/drivers/gpu/drm/drm_atomic_helper.c
index fad2b93..0cd0546 100644
--- a/drivers/gpu/drm/drm_atomic_helper.c
+++ b/drivers/gpu/drm/drm_atomic_helper.c
@@ -1217,8 +1217,8 @@
 
 	return ret;
 backoff:
-	drm_atomic_legacy_backoff(state);
 	drm_atomic_state_clear(state);
+	drm_atomic_legacy_backoff(state);
 
 	/*
 	 * Someone might have exchanged the framebuffer while we dropped locks
@@ -1285,8 +1285,8 @@
 
 	return ret;
 backoff:
-	drm_atomic_legacy_backoff(state);
 	drm_atomic_state_clear(state);
+	drm_atomic_legacy_backoff(state);
 
 	/*
 	 * Someone might have exchanged the framebuffer while we dropped locks
@@ -1462,8 +1462,8 @@
 
 	return ret;
 backoff:
-	drm_atomic_legacy_backoff(state);
 	drm_atomic_state_clear(state);
+	drm_atomic_legacy_backoff(state);
 
 	/*
 	 * Someone might have exchanged the framebuffer while we dropped locks
@@ -1528,8 +1528,8 @@
 
 	return ret;
 backoff:
-	drm_atomic_legacy_backoff(state);
 	drm_atomic_state_clear(state);
+	drm_atomic_legacy_backoff(state);
 
 	goto retry;
 }
@@ -1587,8 +1587,8 @@
 
 	return ret;
 backoff:
-	drm_atomic_legacy_backoff(state);
 	drm_atomic_state_clear(state);
+	drm_atomic_legacy_backoff(state);
 
 	goto retry;
 }
@@ -1646,8 +1646,8 @@
 
 	return ret;
 backoff:
-	drm_atomic_legacy_backoff(state);
 	drm_atomic_state_clear(state);
+	drm_atomic_legacy_backoff(state);
 
 	goto retry;
 }
@@ -1725,8 +1725,8 @@
 
 	return ret;
 backoff:
-	drm_atomic_legacy_backoff(state);
 	drm_atomic_state_clear(state);
+	drm_atomic_legacy_backoff(state);
 
 	/*
 	 * Someone might have exchanged the framebuffer while we dropped locks