fbsplash: use line_length instead of xres * bytes_per_pixel

fbsplash: use line_length instead of xres * bytes_per_pixel

In some system (e.g. my "strange" device), line_length is not equal xres *
bytes_per_pixel, so we need to use line_length in scr_fix to mmap and draw
rectangle etc.

My "strange" device output this:
bytes_per_pixel: 4
xres: 600, yres: 1024
yoffset: 0, xoffset: 0, line_length: 2432
G.addr: b74da000

Signed-off-by: Yin Kangkai <kangkai.yin@intel.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>