drm/i915/dg2: use 128b/132b transcoder DDI mode

128b/132b has a separate transcoder DDI mode, which also requires the
MST transport select to be set. Note that we'll use DP MST also for
single-stream 128b/132b.

Having the FDI and 128b/132b modes share the register mode value
complicates things a bit.

v2:
- Use HAS_DP20 abstraction for 128b/132b mode (Ville)
- Use intel_dp_is_uhbr() helper

Bspec: 50493
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/279bfbd979e0256fae13a5231e07e2f4fb665c07.1631191763.git.jani.nikula@intel.com