drm/xe/gsc: Define GSCCS for MTL

Add the GSCCS to the media_xelpmp engine list. Note that since the
GSCCS is only used with the GSC FW, we can consider it disabled if we
don't have the FW available.

v2: mark GSCCS as allowed on the media IP in kunit tests

Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Alan Previn <alan.previn.teres.alexis@intel.com>
Reviewed-by: John Harrison <John.C.Harrison@Intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>