drm/i915/selftests: Be engine agnostic

When using MI operations, we do not care which engine we use, so use
them all where possible, and where inconvenient double check we have the
engine we selected at random.

v2: Drop the local copy of engine->sseu to avoid an unchecked deref

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190704212343.6820-1-chris@chris-wilson.co.uk