drm/xe/vf: Cache value of the GMDID register

Read and cache value of the GMDID register as part of the config
query that VF driver is doing over MMIO.

While the VF driver likely already obtained the value of the GMDID
register once during the early driver probe, we couldn't cache it
then as the GT structures were not ready yet.

Cache it now, in case the driver needs it later when the GuC MMIO
communication, required to query GMDID from GuC, could be no longer
desired as it will be replaced by the CTB communication.

While around, assert that we will query GMDID only when applicable.

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240523192240.844-6-michal.wajdeczko@intel.com