drm/i915/mtl: don't expose GSC command streamer to the user

There is no userspace user for this CS yet, we only need it for internal
kernel ops (e.g. HuC, PXP), so don't expose it.

v2: even if it's not exposed, rename the engine so it is easier to
identify in the debug logs (Matt)

Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221102171047.2787951-6-daniele.ceraolospurio@intel.com