drm/i915/dp: Implement the POST_LT_ADJ_REQ sequence

drm/i915/dp: Implement the POST_LT_ADJ_REQ sequence

Implement the POST_LT_ADJ_REQ sequence, which should be used
to further fine tune the link if TPS4 is not supported.
The POST_LT_ADJ_REQ sequence will be performed after
the normal link training has succeeded.

Only the final hop between the last LTTPR and DPRX will
perform the POST_LT_ADJ_REQ adjustment. The earlier hops
will use TPS4 instead since it's mandatory for LTTPRs.

The sequence will terminate when the sink clears the
"in progress" flag, the vswing/pre-emphasis values have
changed six times, or the vswing/pre-emphasis values have
remained unchanged for 200 ms.

Tested-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patch.msgid.link/20260330202629.28616-2-ville.syrjala@linux.intel.com