drm/xe/bb: assert width in xe_bb_create_migration_job()

The q->width should always be exactly one here for migration queue/vm.
The width will anyway be overridden later since we need to emit two
jumps for special migration jobs. Enforce that here to ensure caller is
not doing something strange. While here also convert to the helper to
determine if the queue is migration based.

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Nirmoy Das <nirmoy.das@intel.com>
Reviewed-by: Nirmoy Das <nirmoy.das@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240320112730.219854-4-matthew.auld@intel.com