OMAP: DSS2: don't power off a panel twice

If we blank the panel by
echo 1 > /sys/devices/platform/omapfb/graphics/fb0/blank

Then, we reboot the sytem, the kernel will crash at
drivers/video/omap2/dss/core.c:323

This is because the panel is closed twice. Now check the state of a dssdev
to forbid a panel is power on or power off twice.

Signed-off-by: Stanley.Miao <stanley.miao@windriver.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@nokia.com>