drivers/video: fsl-diu-fb: clean up printk usage

Remove debug printk messages (they don't help in debugging), replace
printk(KERN_xxx with its pr_xxx or dev_xxx equivalent, and add a couple
missing error messages.
Signed-off-by: Timur Tabi <timur@freescale.com>
Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>