drm/i915/uc: prefer intel_gt over i915 in GuC/HuC paths

With our HW interface logic moving from i915 to gt and with GuC and HuC
being part of the gt HW, it makes sense to use the intel_gt structure
instead of i915 as our reference object in GuC/HuC paths.

Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190713100016.8026-9-chris@chris-wilson.co.uk
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>