drm-uapi/xe_drm: Add Xe uAPI

Introduce uAPI contract for Xe IGT.

Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: Philippe Lecluse <philippe.lecluse@intel.com>
Signed-off-by: Zbigniew Kempczyński <zbigniew.kempczynski@intel.com>
Acked-by: Mauro Carvalho Chehab <mchehab@kernel.org>