drivers/video: fsl-diu-fb: the video buffer is not I/O memory

The video buffer is not uncached memory-mapped I/O, so don't tag the virtual
address as __iomem.  It's also not a u8*.

Signed-off-by: Timur Tabi <timur@freescale.com>
Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>