drm/i915/guc: Use the new gt_to_guc() wrapper

Get the guc reference from the gt using the gt_to_guc() helper.

Signed-off-by: Andi Shyti <andi.shyti@linux.intel.com>
Reviewed-by: Nirmoy Das <nirmoy.das@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231229102734.674362-3-andi.shyti@linux.intel.com