drm/i915: Remove locking for get-caching query

Reading a single value from the object, the locking only provides futile
protection against userspace races. The locking is useless so remove it.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>