video: s3c-fb: add alpha value width setting

This patch adds alpha value width setting according to
transparency value of each window format, using BLENDCON
register. When alpha value is 8 bits, BLENDCON will set
alpha value width as 8 bits.

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>