drm/i915: Skip the execlists CSB scan and rewrite if the ring is untouched

If the CSB head/tail pointers are unchanged, we can skip the update of
the CSB register afterwards.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170124110009.28947-5-chris@chris-wilson.co.uk