drm/i915: set proper DPIO post divider for VGA on VLV v4
Supposedly we should use the DAC divider for <300MHz pixel clocks, but as
that doesn't actually work as well as the high freq divider here in
practice, just use the high freq divider all the time.
v2: remove unconditional write (Jesse)
check for pixel rate properly (Jesse)
v3: give up, the DAC divider apparently doesn't work, and low res modes
work ok (Jesse)
remove debug msg (Jesse)