drm/i915/selftests: Count the number of engines used

Don't rely on the RUNTIME_INFO() when we loop over a particular context
and only run on a filtered set of engines.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191127223252.3777141-1-chris@chris-wilson.co.uk