drm/i915: Remove obsolete engine cleanup

Remove the outer layer cleanup of engine stubs; as i915_drv itself no
longer tries to preallocate and so is not responsible for either the
allocation or free. By the time we call the cleanup function, we already
have cleaned up the engines.

v2: Lack of symmetry between mmio_probe and mmio_release for handling
the error cleanup. engine->destroy() is a compound function that is
called earlier in the normal release as it ties together other bits of
state.

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/20190718070024.21781-6-chris@chris-wilson.co.uk