drm/i915/dp_mst: Reprobe the MST topology after a link parameter change

drm/i915/dp_mst: Reprobe the MST topology after a link parameter change

The MST link BW reported by branch devices via the ENUM_PATH_RESOURCES
message depends on the channel coding and link rate/lane count
parameters programmed to DPCD. This is the case at least for some branch
devices, while for others the reported BW is independent of the link
parameters. In any case the DP standard requires the branch device to
adjust the returned value to both account for the different way the BW
for FEC is accounted for (included in the returned value for non-UHBR
and not included for UHBR rates) and to limit the returned value to the
(trained) link BW between the source and first downstream branch
device, see DP v2.0/v2.1 Figure 2-94, DP v2.1 5.9.7. Presumedly this is
also the reason why the standard requires the DPCD link rate/lane count
values being up-to-date before sending the ENUM_PATH_RESOURCES message,
see DP v2.1 2.14.9.4.

Based on the above reprobe the MST topology after the link is retrained
with new link parameters to make sure that the MST link BW tracked in
the MST topology state (via each topology port's full_pbn value) is
up-to-date.

The next patch will make sure that the MST link BW is also kept
up-to-date if the link is disabled.

Reviewed-by: Suraj Kandpal <suraj.kandpal@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240722165503.2084999-13-imre.deak@intel.com