drm/xe/vm: Avoid asid lookup if none allocated

The destroy path can and will get called for incomplete
vm objects on error paths, where the asid is not yet allocated.
This leads to lookup fail and assert triggered.

Fix this by not asserting of asid existence if vm never
got assigned one.

Cc: Ohad Sharabi <osharabi@habana.ai>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Cc: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>