drm/i915/gt: Create the gt_to_guc() wrapper

We already have guc_to_gt() and getting to guc from the GT it
requires some mental effort. Add the gt_to_guc().

Given the reference to the "gt", the gt_to_guc() will return the
pinter to the "guc".

Update all the files under the gt/ directory.

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/20231229102734.674362-2-andi.shyti@linux.intel.com