drm/i915: Use intel_engine_lookup_user for probing HAS_BSD etc

Use the same mechanism to determine if a backend engine exists for a
uabi mapping as used internally.

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/20190807083702.16349-2-chris@chris-wilson.co.uk