drm/i915/selftest: Cope with not having an RCS engine

It is no longer guaranteed that there will always be an RCS engine.
So, use the helper function for finding the first available engine that
can be used for general purpose selftets.

Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220728024225.2363663-5-John.C.Harrison@Intel.com