drm/i915/selftests: Hold ref on request across waits

As we wait upon the request, we should be sure to hold our own reference
for our checks.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190625130128.11009-14-chris@chris-wilson.co.uk