fbdev: kill sparse warning in deferred IO

Kill the following in fb_defio.c:

drivers/video/fb_defio.c:40:43: warning: incorrect type in argument 1 (different address spaces)
drivers/video/fb_defio.c:40:43:    expected void *addr
drivers/video/fb_defio.c:40:43:    got char [noderef] *screen_base<asn:2>

The framebuffer memory of these types of devices are allocated from system
RAM.

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>