[PATCH] rivafb: use constants instead of magic values

use (MIN/MAX)_LEVEL and FB_(UN)BLANK instead of the values they are defined to

Signed-off-by: Guido Guenther <agx@sigxcpu.org>
Signed-off-by: Antonino Daplas <adaplas@pol.net>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>