rivafb: fix IO access

Fix IO access, with the following sparse warnings:

drivers/video/riva/fbdev.c:320:25: warning: dereference of noderef expression
drivers/video/riva/fbdev.c:321:29: warning: dereference of noderef expression
drivers/video/riva/fbdev.c:327:18: warning: dereference of noderef expression
drivers/video/riva/fbdev.c:328:15: warning: dereference of noderef expression

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>