drm/i915/guc: Create the guc_to_i915() wrapper

Given a reference to "guc", the guc_to_i915() returns the
pointer to "i915" private data.

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/20231206184322.57111-1-andi.shyti@linux.intel.com