drm/i915: take struct_mutex in i915_dma_cleanup()

intel_cleanup_ring_buffer() calls drm_gem_object_unreference() (as
opposed to drm_gem_object_unreference_unlocked()) so it needs to be
called with "struct_mutex" held.  If we don't hold the lock, it triggers
a BUG_ON(!mutex_is_locked(&dev->struct_mutex));

I also audited the other places that call intel_cleanup_ring_buffer()
and they all hold the lock so they're OK.

This was introduced in: 8187a2b7 "drm/i915: introduce
intel_ring_buffer structure (V2)" and it's a regression from v2.6.34.

Addresses: https://bugzilla.kernel.org/show_bug.cgi?id=16247



Signed-off-by: Dan Carpenter <error27@gmail.com>
Reported-by: Benny Halevy <bhalevy@panasas.com>
Tested-by: Benny Halevy <bhalevy@panasas.com>
Signed-off-by: Eric Anholt <eric@anholt.net>