drm/i915/gt: document with_intel_gt_pm_if_awake()

Add a kernel-doc markup to document this new macro.

Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>
Signed-off-by: Andi Shyti <andi.shyti@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/b974905bd0f6b5308b91561cc85eeecd94f1452a.1658924372.git.mchehab@kernel.org