drm/i915: Drop mutex after successful kref_put_mutex()

The kref_put_mutex() returns with the mutex held after freeing the
object - so we must remember to drop it...

Fixes: 69df05e1

 ("drm/i915: Simplify releasing context reference")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161219101357.28140-1-chris@chris-wilson.co.uk
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>