drm/i915/gt: Mark up virtual engine uabi_instance

drm/i915/gt: Mark up virtual engine uabi_instance

Be sure to initialise the uabi_instance on the virtual engine to the
special invalid value, just in case we ever peek at it from the uAPI.

Reported-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Fixes: 750e76b4f9f6 ("drm/i915/gt: Move the [class][inst] lookup for engines onto the GT")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: <stable@vger.kernel.org> # v5.4+
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200106123921.2543886-1-chris@chris-wilson.co.uk