drm/xe/vf: Don't support gtidle if VF

VF drivers can't access any of gtidle control registers as this
functionality is owned by the PF driver.

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Reviewed-by: Piotr Piórkowski <piotr.piorkowski@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240619214557.905-7-michal.wajdeczko@intel.com