drm/xe: fix submissions without vm

drm/xe: fix submissions without vm

Kernel queues can submit privileged batches directly in GGTT, so they
don't always need a vm. The submission front-end already supports
creating and submitting jobs without a vm, but some parts of the
back-end assume the vm is always there. Fix this by handling a lack of
vm in the back-end as well.

v2: s/XE_BUG_ON/XE_WARN_ON, s/engine/exec_queue

Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Link: https://lore.kernel.org/r/20230822173334.1664332-2-daniele.ceraolospurio@intel.com
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>