drm/msm: fix locking inconsistency for gpu->hw_init()

Most, but not all, paths where calling the with struct_mutex held.  The
fast-path in msm_gem_get_iova() (plus some sub-code-paths that only run
the first time) was masking this issue.

So lets just always hold struct_mutex for hw_init().  And sprinkle some
WARN_ON()'s and might_lock() to avoid this sort of problem in the
future.

Signed-off-by: Rob Clark <robdclark@gmail.com>

drivers/gpu/drm/msm/msm_gpu.c

@@ -203,6 +203,8 @@ int msm_gpu_hw_init(struct msm_gpu *gpu)
 {
 	int ret;
 
+	WARN_ON(!mutex_is_locked(&gpu->dev->struct_mutex));
+
 	if (!gpu->needs_hw_init)
 		return 0;