drm/i915: Drop final few uses of drm_i915_private.engine

We've migrated all the heavy users over to the intel_gt, and can finally
drop the last few users and with that the mirror in dev_priv->engine[].

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Andi Shyti <andi.shyti@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200325234803.6175-1-chris@chris-wilson.co.uk