drm/i915/execlists: Assign virtual_engine->uncore from first sibling

Copy across the engine->uncore shortcut to the virtual_engine from its
first physical engine, similar to the handling of the engine->gt
backpointer.

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/20191008070342.4045-1-chris@chris-wilson.co.uk