drm/i915/gt: Incorporate the virtual engine into timeslicing

It was quite the oversight to only factor in the normal queue to decide
the timeslicing switch priority. By leaving out the next virtual request
from the priority decision, we would not timeslice the current engine if
there was an available virtual request.

Testcase: igt/gem_exec_balancer/sliced
Fixes: 3df2deed

 ("drm/i915/execlists: Enable timeslice on partial virtual engine dequeue")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200519132046.22443-3-chris@chris-wilson.co.uk