drm/i915: Do interrupible mutex lock first to avoid locking for unreference

One of the primarily consumers of the i915 driver is X, a large signal
driven application. Frequently when writing into the buffers, there is a
pending signal which causes us not to take the interruptible lock but
then we need to take that same lock around the object unreference. By
rearranging the code to do the interruptible lock as the first check, we
can avoid the frequent additional locking around the unreference.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>