drm/i915: fix deadlock in fb teardown

At module unload time we'll tear down the fbdev state.  We do so under
the struct mutex, so we shouldn't try to use the unlocked variant of
the GEM object unreference function or we may deadlock.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Eric Anholt <eric@anholt.net>