vga16fb: actually support widths in multiples of 8

vga16fb does not just support 8-pixel wide rectangles, but rectangles with
widths in multiples of 8.

Signed-off-by: Antonino Daplas <adaplas@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>