drm/radeon/kms: enable frac fb divs on rs600/rs690/rs740

Allows us to hit dot clocks much closer, especially on
chips with non-27 Mhz reference clocks like most IGP chips.
This fixes most flickering and blanking problems with
non-exact dot clocks on these chips.

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>