video: fbdev: imxfb: Drop unused symbols from header

The only file that includes <linux/platform_data/video-imxfb.h> is the
imxfb driver. Drop all symbols that are unused there.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Helge Deller <deller@gmx.de>