fbdev: sh_mobile_lcdc: Fix vertical panning step

Commit 15dede88 added support for
horizontal panning but accidentally computes the Y pan step value
incorrectly for NV12/21 and NV16/61 formats. Fix this.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>