vfb: use sys instead of cfb drawing functions

Since vfb's framebuffer is vmalloc'ed, use the sys_* drawing functions
instead of cfb_*.

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>