da8xx-fb: add missing FB_BLANK operations

add FB_BLANK_NORMAL, FB_BLANK_VSYNC_SUSPEND and FB_BLANK_HSYNC_SUSPEND
modes (copy drivers/video/omap2/omapfb/omapfb-main.c implementation).
Otherwise X-server will complain about invalid parameter.

Signed-off-by: Yegor Yefremov <yegorslists@googlemail.com>
Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>