drm/i915/dp: Use a commit modeset for link retraining MST links

drm/i915/dp: Use a commit modeset for link retraining MST links

Instead of direct calls to the link train functions, retrain the link
via a commit modeset. The direct call means that the output port will be
disabled/re-enabled while the rest of the pipeline (transcoder) is
active, which doesn't seem to work on MST at least. It leads to
underruns and black screen, presumedly because the transcoder is not
disabled/re-enabled along the port.

Leave switching to a commit modeset on SST for a later patchset, as that
seems to work ok currently (though better to using a commit there too,
due to the suppressed underruns).

v2: Keep reverse line length order for local variables. (Ville)

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240610164933.2947366-7-imre.deak@intel.com