drm/i915: Hold struct_mutex while unreffing pwrctx object

This also extends the mutex to cover fbc disabling, which is safe.

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