drm/i915: Make engine's batch pool safe for use with virtual engines

A virtual engine itself does not have a batch pool, but we can gleefully
use any of its siblings instead.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190827135935.3831-1-chris@chris-wilson.co.uk