drm/amd/display: Retrain dongles when SINK_COUNT becomes non-zero

drm/amd/display: Retrain dongles when SINK_COUNT becomes non-zero

[WHY]
Two years ago the patch referenced by the Fixes tag stopped running
dp_verify_link_cap_with_retries during DP detection when the reason
for the detection was a short-pulse interrupt. This effectively meant
that we were no longer doing the verify_link_cap training on active
dongles when their SINK_COUNT changed from 0 to 1.

A year ago this was partly remedied with:
commit 43686cca57dc ("drm/amd/display: Don't skip link training for empty dongle")

This made sure that we trained the dongle on initial hotplug (without
connected downstream devices).

This is all fine and dandy if it weren't for the fact that there are
some dongles on the market that don't like link training when SINK_COUNT
is 0 These dongles will in fact indicate a SINK_COUNT of 0 immediately
after hotplug, even when a downstream device is connected, and then
trigger a shortpulse interrupt indicating a SINK_COUNT change to 1.

In order to play nicely we will need our policy to not link train an
active DP dongle when SINK_COUNT is 0 but ensure we train it when the
SINK_COUNT changes to 1.

[HOW]
Call dp_verify_link_cap_with_retries on detection even when the detection
is triggered from a short pulse interrupt.

With this change we can also revert this commit which we'll do in a separate
follow-up change:
commit 43686cca57dc ("drm/amd/display: Don't skip link training for empty dongle")

Fixes: 3c31b8ee4576 ("drm/amd/display: DP Compliance 400.1.1 failure")
Suggested-by: Louis Li <Ching-shih.Li@amd.com>
Tested-by: Louis Li <Ching-shih.Li@amd.com>
Cc: Wenjing Liu <Wenjing.Liu@amd.com>
Cc: Hersen Wu <hersenxs.wu@amd.com>
Cc: Eric Yang <Eric.Yang2@amd.com>
Reviewed-by: Wenjing Liu <Wenjing.Liu@amd.com>
Signed-off-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>