drm/i915/selftests: Take an explicit ref for rq->batch

Since we are peeking into the batch object of the request, it is
beholden on us to hold a reference to it.

Closes: https://gitlab.freedesktop.org/drm/intel/issues/1634


Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200408091723.28937-1-chris@chris-wilson.co.uk