video: fbdev: imxfb: Fold <linux/platform_data/video-imxfb.h> into only user

No source file but the driver itself includes the header containing the
platform data definition. The last user is gone since commit
8485adf1

 ("ARM: imx: Remove imx device directory").

Move the remaining symbols directly into the driver and remove the then
unused header file.

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