drm/i915/gt: add gt_is_root() helper

The "gt_is_root(struct intel_gt *gt)" helper return true if the
gt is the root gt, which means that its id is 0. Return false
otherwise.

Suggested-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Signed-off-by: Andi Shyti <andi.shyti@linux.intel.com>
Reviewed-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Reviewed-by: Andrzej Hajda <andrzej.hajda@intel.com>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220318233938.149744-3-andi.shyti@linux.intel.com