drm/xe/vf: Support only GuC/HuC firmwares

Only GuC/HuC firmwares can be used by the VFs, don't claim support
for any other firmware (like GSC) even if the driver can support it.

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Reviewed-by: Piotr Piórkowski <piotr.piorkowski@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240604212231.1416-2-michal.wajdeczko@intel.com