[PATCH] rivafb: fix initial brightness

This is the rivafb equivalent of 238576e1

.
It fixes rivafb having a default backlight brightness of 0 (no picture at
all) on a PBook 6,1.

Signed-off-by: Guido Guenther <agx@sigxcpu.org>
Cc: "Antonino A. Daplas" <adaplas@pol.net>
Acked-by: Richard Purdie <rpurdie@rpsys.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>