drm/i915/selftests: Teach requests to use all available engines

The request selftests straddle the boundary between checking the driver
and the hardware. They are subject to the quirks of the underlying HW,
but operate on top of the backend abstractions. The tests focus on the
scheduler elements and so should check for interactions of the scheduler
across all exposed engines.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191016125236.17960-1-chris@chris-wilson.co.uk