fbdev: add sparse annotations in svgalib.c

Add sparse annotations and use fb_read/fb_write family to access the
framebuffer.

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>