Missing ctx_count decrement when releasing driver.

From: Erdi Chen
Signed-off-by: Dave Airlie <airlied@linux.ie>

drivers/char/drm/drm_drv.h

@@ -836,6 +836,7 @@ int DRM(release)( struct inode *inode, struct file *filp )
 
 				list_del( &pos->head );
 				DRM(free)( pos, sizeof(*pos), DRM_MEM_CTXLIST );
+				--dev->ctx_count;
 			}
 		}
 	}