drm/xe: Do not attempt to bootstrap VF in execlists mode

It was mentioned in a review that there is a possibility of choosing
to load the module with VF in execlists mode.

Of course this doesn't work, just bomb out as hard as possible.

Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241210083111.230484-12-dev@lankhorst.se


Signed-off-by: Maarten Lankhorst <dev@lankhorst.se>