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

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>