drm/vmwgfx: Fix recently introduced sparse / smatch warnings and errors

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Jakob Bornecrant <jakob@vmware.com>
diff --git a/drivers/gpu/drm/vmwgfx/vmwgfx_surface.c b/drivers/gpu/drm/vmwgfx/vmwgfx_surface.c
index 3bb3331..979da1c 100644
--- a/drivers/gpu/drm/vmwgfx/vmwgfx_surface.c
+++ b/drivers/gpu/drm/vmwgfx/vmwgfx_surface.c
@@ -1117,6 +1117,7 @@
 	if (unlikely(cmd == NULL)) {
 		DRM_ERROR("Failed reserving FIFO space for surface "
 			  "destruction.\n");
+		mutex_unlock(&dev_priv->binding_mutex);
 		return -ENOMEM;
 	}