drm/msm/dp: Introduce link training per-segment for LTTPRs

DisplayPort requires per-segment link training when LTTPR are switched
to non-transparent mode, starting with LTTPR closest to the source.
Only when each segment is trained individually, source can link train
to sink.

Implement per-segment link traning when LTTPR(s) are detected, to
support external docking stations. On higher level, changes are:

* Pass phy being trained down to all required helpers
* Run CR, EQ link training per phy
* Set voltage swing, pre-emphasis levels per phy

Since at least some LTTPRs (eg. Parade PS8830) do not correctly report
voltage-swing, pre-emphasis level 3 support, always assume level 3 is
supported. This is permitted under DP 2.1(a) section 3.6.7.2 stating
that LTTPR shall set its transmitter levels as close as possible to
those requested by the DPTX, if the DPTX sets the voltage swing or
pre-emphasis to a level that the LTTPR does not support. It shall be
noted that LTTPR’s level choosing is implementation-specific.

This ensures successful link training both when connected directly to
the monitor (single LTTPR onboard most X1E laptops) and via the docking
station (at least two LTTPRs).

Fixes: 72d0af4accd9 ("drm/msm/dp: Add support for LTTPR handling")
Tested-by: Jessica Zhang <[email protected]> # SA8775P
Tested-by: Johan Hovold <[email protected]>
Tested-by: Rob Clark <[email protected]>
Tested-by: Stefan Schmidt <[email protected]>
Signed-off-by: Aleksandrs Vinarskis <[email protected]>
Reviewed-by: Abel Vesa <[email protected]>
Reviewed-by: Abhinav Kumar <[email protected]>
Reviewed-by: Dmitry Baryshkov <[email protected]>
Patchwork: https://patchwork.freedesktop.org/patch/652305/
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Dmitry Baryshkov <[email protected]>

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