drm/i915/dp: Retrain SST links via a modeset commit

Instead of direct calls of the link training functions, use a modeset
commit to retrain a DP link in SST mode, similarly to how this is done
in DP-MST mode. Originally the current way was chosen presumedly,
because there wasn't a well-established way in place for the driver to
do an internal (vs. userspace/kernel client) commit. Since then such
internal commits became a common place (initial-, HDMI/TC link reset
commit), so there is no reason to handle the DP-SST link-retraining case
differently.

At the end of the current sequence the HW reported a FIFO underrun -
without other issues visible to users - because during retraining the
link's encoder/port was disabled/re-enabled without also
disabling/re-enabling the corresponding pipe/transcoder (as required by
the spec); the corresponding underrun error message was suppressed as a
known issue. Based on Ankit's test on DG2 the underrun error was still
reported as it got detected with some (vblank) delay wrt. other
platforms. Switching to a modeset commit resolves these underrun related
issues.

Cc: Ankit Nautiyal <[email protected]>
Cc: Ville Syrjälä <[email protected]>
Reviewed-by: Suraj Kandpal <[email protected]>
Signed-off-by: Imre Deak <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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