drm/i915: kill intel_clear_scanline_wait

This is a pretty racy way to close these races, and we have
much better means to cope with these races meanwhile: For
non-broken userspace we correctly wait for any outstanding
rendering, for broken userspace the hangcheck will save the
day.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>