drm/mxsfb: Fix initial corrupt frame when activating display

LCDIF will repeatedly display data from CUR_BUF and set CUR_BUF to
NEXT_BUF when done. Since we are only ever writing to NEXT_BUF the
display will show an initial corrupt frame.

Fix by writing the FB paddr to both CUR_BUF and NEXT_BUF when
activating the CRTC.

Signed-off-by: Leonard Crestez <leonard.crestez@nxp.com>
Tested-by: Philipp Zabel <p.zabel@pengutronix.de>
Reviewed-by: Stefan Agner <stefan@agner.ch>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Stefan Agner <stefan@agner.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/7cdac9c064cc2b8a3d237934f186da98cefe6cb3.1537191359.git.leonard.crestez@nxp.com