drm/i915: Sanitize VLV/CHV watermarks properly

Clear out the watermark for all disabled planes to 0. This is required
to avoid falsely thinking that the inherited watermarks are bogus in
case the watermark is actually higher than the FIFO size.

v2: s/noninverted/raw/ for consistency with other platforms

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170302171508.1666-13-ville.syrjala@linux.intel.com