drm/xe/pf: Drop GuC notifications for non-existing VF

It is unlikely that GuC will ever send a G2H notification with an
invalid VFID and it is currently harmless if that actually happen.
But in upcoming patches we will start using that VFID as an index
and we must be sure it is a valid to avoid a crash due to a buggy
firmware or a currupted G2H message.

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Reviewed-by: Piotr Piórkowski <piotr.piorkowski@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240828210809.1528-4-michal.wajdeczko@intel.com