nvidiafb: Add proper support for Geforce 7600 chipset

Add proper support for the Geforce 7600 (device id 0x039x).  This also sync's
nvidiafb with the latest Xorg nv driver.

Signed-off-by: Antonino Daplas <adaplas@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>