atyfb: halve XCLK with Mobility and 32bit memory

Laptops with Rage Mobility and 32bit memory interface seem to require halved
XCLK to operate correctly.

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>