radeonfb: fix setting of PPLL_REF_DIV for RV370 5B60.

As observed with various Radeon X300 cards console goes blank
without that fix.

Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Andreas Herrmann <aherrman@arcor.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>