drm/i915: Refactor common lock handling between shrinker count/scan

We can share a few lines of tricky lock handling we need to use for both
shrinker routines and in the process fix the return value for count()
when reporting a deadlock.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Robert Beckett <robert.beckett@intel.com>
Reviewed-by: Rafael Barbalho <rafael.barbalho@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>