drm/i915/guc: prefer intel_gt in guc interrupt functions

We can get rid of a few more guc_to_i915 and start compartmentalizing
interrupt management a bit more. We should be able to move more code in
the future once the gt_pm code is also moved across to gt.

Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190713100016.8026-10-chris@chris-wilson.co.uk
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>