video: s3c-fb: add additional validate bpps

Additional validate bpps are added to windows as follows:
window0: 18 bpp for RGB666
window1, 2 and 3: 28 bpp for ARGB4888
window4: 19 bpp for ARGB1666, 28 bpp for ARGB4888

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>