drm/i915/ats-m: add ATS-M platform info

ATS-M is a server platform based on Xe_HPG and Xe_HPM, but without
display support.  From a driver point of view, it's easiest to just
handle it as DG2 (including identifying as PLATFORM_DG2), but with the
display disabled in the device info.

Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Balasubramani Vivekanandan <balasubramani.vivekanandan@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220329000822.1323195-1-matthew.d.roper@intel.com