drm/i915: Local debug BUG_ON for intel_wakeref

Avoid pulling in i915_gem.h just so that we can use a conditional BUG_ON
for debugging.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190621183801.23252-5-chris@chris-wilson.co.uk