drm/i915: Make pm_notify take intel_gt

These notifications operate on intel_gt so make the code take what it
needs.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Andi Shyti <andi.shyti@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190910143823.10686-5-tvrtko.ursulin@linux.intel.com