savagefb: fix DDC for Savage 4

savagefb: fix DDC for Savage 4

I tested savagefb on 3 different Savage 4 cards:
Diamond Stealth III S520
Number Nine SR9
Datapath Horizon 2S (two savage chips on a PCI card)

it worked except the DDC which did not work on any of them.

Looking at the BIOS code, it does not use MMIO register 0xff20 but CRT
register 0xa0 or 0xb1 - depending on the chip revision and something in
register 0xa6.  With this patch, DDC works fine on all 3 cards (even on
the second head of Horizon 2S - although it does not display anything as
it's misconfigured because of missing BIOS).

Signed-off-by: Ondrej Zary <linux@rainbow-software.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>