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: NAntonino Daplas <adaplas@gmail.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>