fbdev: sh_mobile_meram: Fix MExxCTL register save on runtime PM suspend

To reset the ICB on resume the MExxCTL register needs to be OR'ed with
MExxCTL_WBF | MExxCTL_WF | MExxCTL_RF, no set to that value. Fix this.

This fixes corruption at the bottom of the display when resuming from
runtime PM.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>