video: stop sh_mobile_lcdcfb only if started

This patch fixes the LCDC driver to avoid calling the
function sh_mobile_lcdc_start_stop(priv, 0) unless the
same function has been called before to start the LCDC
hardware.

Triggered when sh_mobile_lcdcfb.c failed to probe() due to
missing MSTP clocks.

Signed-off-by: Magnus Damm <damm@igel.co.jp>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>