drm/i915/dp: compute the pch dp aux divider from the rawclk

Otherwise dp aux won't work on some hsw platforms, since they use a
different rawclk than the 125MHz clock used thus far.

To absolutely not change anything, round up: That way we get the old
63 divider for the default 125MHz clock.

Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Tested-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>