drm/i915: Adjust DisplayPort clocks to use 96MHz reference

For some reason, the DP clocks were based off a 100MHz reference instead of
the standard 96MHz reference. This caused some DP monitors to fail to lock
to the signal.

Signed-off-by: Keith Packard <keithp@keithp.com>
Signed-off-by: Eric Anholt <eric@anholt.net>