drm/i915/gt: Skip over completed active execlists, again

Now that we are careful to always force-restore contexts upon rewinding
(where necessary), we can restore our optimisation to skip over
completed active execlists when dequeuing.

Referenecs: 35f3fd81 ("drm/i915/execlists: Workaround switching back to a completed context")
References: 8ab3a381 ("drm/i915/gt: Incrementally check for rewinding")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210120121718.26435-2-chris@chris-wilson.co.uk