drm/i915: Re-apply "Perform link quality check, unconditionally during long pulse"

This re-applies the workaround for "some DP sinks, [which] are a
little nuts" from commit 1a36147b ("drm/i915: Perform link
quality check unconditionally during long pulse").
It makes the secondary AOC E2460P monitor connected via DP to an
acer Veriton N4640G usable again.

This hunk was dropped in commit c85d200e ("drm/i915: Move SST
DP link retraining into the ->post_hotplug() hook")

Fixes: c85d200e

 ("drm/i915: Move SST DP link retraining into the ->post_hotplug() hook")
[Cleaned up commit message, added stable cc]
Signed-off-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Jan-Marek Glogowski <glogow@fbihome.de>
Cc: stable@vger.kernel.org
Link: https://patchwork.freedesktop.org/patch/msgid/20180825191035.3945-1-lyude@redhat.com