drm/i915/selftests: Free the batch along the contexts error path

Remember to release the batch bo reference if we hit an error trying to
submit our MI_STORE_DWORD_IMM.

References: https://bugs.freedesktop.org/show_bug.cgi?id=107979
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/20180919195544.1511-9-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/selftests/i915_gem_context.c

@@ -195,6 +195,7 @@ err_request:
 	i915_request_add(rq);
 err_batch:
 	i915_vma_unpin(batch);
+	i915_vma_put(batch);
 err_vma:
 	i915_vma_unpin(vma);
 	return err;