drm/i915/ddi: initialize 128b/132b SST DP2 VFREQ registers

Write the DP2 specific VFREQ registers.

This is preparation for enabling 128b/132b SST. This path is not
reachable yet.

v2: Check for !is_hdmi (Imre)

Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/7d90547e9ce01642b722efca0bf81cadb754e790.1735912293.git.jani.nikula@intel.com