drm/i915/ivb: Move WaCxSRDisabledForSpriteScaling w/a to atomic check

Determine whether we need to apply this workaround at atomic check time
and just set a flag that will be used by the main watermark update
routine.

Moving this workaround into the atomic framework reduces
ilk_update_sprite_wm() to just a standard watermark update, so drop it
completely and just ensure that ilk_update_wm() is called whenever a
sprite plane is updated in a way that would affect watermarks.

Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>