Revert "drm/i915: Do not rely on wm preservation for ILK watermarks"

This reverts commit 7de69478 which is
commit 8777b927 upstream.

It was reported to cause flickering and other regressions.

Reported-by: Rainer Fiebig <jrf@mailbox.org>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Matt Roper <matthew.d.roper@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

reverted: