drm/i915/execlists: Track active elements during dequeue

drm/i915/execlists: Track active elements during dequeue

Record the initial active element we use when building the next ELSP
submission, so that we can compare against it latter to see if there's
no change.

Fixes: a99d4c53fd08 ("drm/i915/execlists: Skip redundant resubmission")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200311092624.10012-2-chris@chris-wilson.co.uk
(cherry picked from commit 60ef5b7ac6a131f09d287a5f156c878c2c926a30)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>