drm/i915/xelpmp: Expose media as another GT

Xe_LPM+ platforms have "standalone media." I.e., the media unit is
designed as an additional GT with its own engine list, GuC, forcewake,
etc. Let's allow platforms to include media GTs in their device info.

v2:
- Simplify GSI register handling and split it out to a separate patch
for ease of review. (Daniele)

Cc: Aravind Iddamsetty <aravind.iddamsetty@intel.com>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Aravind Iddamsetty <aravind.iddamsetty@intel.com>
Acked-by: Aravind Iddamsetty <aravind.iddamsetty@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220906234934.3655440-13-matthew.d.roper@intel.com
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>