drm/xe: Add media GT to tile

This media_gt pointer isn't actually allocated yet.  Future patches will
start hooking it up at appropriate places in the code, and then creation
of the media GT will be added once those infrastructure changes are in
place.

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://lore.kernel.org/r/20230601215244.678611-20-matthew.d.roper@intel.com


Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>