drm/i915/selftests: Hold request reference over waits

Take a reference on the request before submitting it to the HW and then
waiting on it for selftest_workarounds. Once submitted, the request may
be freed by a background worker, unless we take an extra reference for
ourselves.

References: https://bugs.freedesktop.org/show_bug.cgi?id=111926


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/20191009061759.3189-1-chris@chris-wilson.co.uk