drm/i915/ptl/dp_mst: Fix slave transcoder enabling wrt. DDI function

On PTL 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.

This fixes a slave transcoder getting stuck during enabling, leading
to page flip timeout errors on the corresponding pipe.

The spec requires the same programming step for ADLP+ platforms, that
will be addressed separately (on those platforms the above transcoder
getting stuck issue was not observed).

Bspec: 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/20241030192313.4030617-2-imre.deak@intel.com