drm/i915/adlp+/dp_mst: Align slave transcoder enabling with spec wrt. DDI function

drm/i915/adlp+/dp_mst: Align slave transcoder enabling with spec wrt. DDI function

On ADLP+ during modeset enabling configure the DDI function without
enabling it for MST slave transcoders before programming the data and
link M/N values. The DDI function gets enabled separately later in the
transcoder enabling sequence.

Align the code with the spec based on the above.

v2: Move this patch earlier in the series, addressing the DP2
    config fixes for all ADLP+ platforms later.

Bspec: 55424, 54128, 65448, 68849
Reviewed-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241030192313.4030617-3-imre.deak@intel.com