video: s3c-fb: fix red offset and length for ARGB232 format

Green pixel and blue pixel are 3 bits and 2 bits respectively
at ARGB232 format. Thus, the value of red offset should be 5,
not 4. Also, the value of red length should be 2, because
red pixel is 2 bits at ARGB232 format.

Signed-off-by: Jingoo Han <jg1.han@samsung.com>