drm/i915/dp: Fix disabling the transcoder function in 128b/132b mode

drm/i915/dp: Fix disabling the transcoder function in 128b/132b mode

During disabling the transcoder in DP 128b/132b mode (both in case of an
MST master transcoder and in case of SST) the transcoder function must
be first disabled without changing any other field in the register (in
particular leaving the DDI port and mode select fields unchanged) and
clearing the DDI port and mode select fields separately, later during
the disabling sequences. Fix the sequence accordingly.

Bspec: 54128, 65448, 68849
Cc: Jani Nikula <jani.nikula@intel.com>
Fixes: 79a6734cd56e ("drm/i915/ddi: disable trancoder port select for 128b/132b SST")
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250217223828.1166093-3-imre.deak@intel.com
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>