drm/i915/xehp: Define compute class and engine

Introduce a Compute Command Streamer (CCS), which has access to
the media and GPGPU pipelines (but not the 3D pipeline).

To begin with, define the compute class/engine common functions, based
on the existing render ones.

v2:
 - Add kerneldoc for drm_i915_gem_engine_class since we're adding a new
   element to it.  (Daniel)
 - Make engine class <-> guc class converters use lookup tables to make
   it more clear/explicit how the IDs map.  (Tvrtko)

v3:
 - Don't update uapi for now; we'll just include the driver-internal
   changes for the time being.

Bspec: 46167, 45544
Original-author: Michel Thierry
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Cc: Vinay Belgaumkar <vinay.belgaumkar@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Signed-off-by: Aravind Iddamsetty <aravind.iddamsetty@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com> #v1
Link: https://patchwork.freedesktop.org/patch/msgid/20220301231549.1817978-2-matthew.d.roper@intel.com