drm/i915/xehp: Add compute engine ABI

We're now ready to start exposing compute engines to userspace.

v2:
 - Move kerneldoc for other engine classes to a separate patch.  (Andi)

Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Cc: Vinay Belgaumkar <vinay.belgaumkar@intel.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Szymon Morek <szymon.morek@intel.com>
UMD (mesa): https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/14395
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Acked-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Tested-by: Jordan Justen <jordan.l.justen@intel.com>  # mesa anvil & iris
Link: https://patchwork.freedesktop.org/patch/msgid/20220428041926.1483683-4-matthew.d.roper@intel.com