drm/ast: Fixed 1280x800 Display Issue

The original ast driver cannot display properly if the resolution is 1280x800 and the pixel clock is 83.5MHz.
Here is the update to fix it.

Signed-off-by: Y.C. Chen <yc_chen@aspeedtech.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>