drm/i915: Move SST DP link retraining into the ->post_hotplug() hook

Doing link retraining from the short pulse handler is problematic since
that might introduce deadlocks with MST sideband processing. Currently
we don't retrain MST links from this code, but we want to change that.
So better to move the entire thing to the hotplug work. We can utilize
the new encoder->hotplug() hook for this.

The only thing we leave in the short pulse handler is the link status
check. That one still depends on the link parameters stored under
intel_dp, so no locking around that but races should be mostly harmless
as the actual retraining code will recheck the link state if we
end up there by mistake.

v2: Rebase due to ->post_hotplug() now being just ->hotplug()
    Check the connector type to figure out if we should do
    the HDMI thing or the DP think for DDI

[pushed with whitespace changes for sparse]
Cc: Manasi Navare <manasi.d.navare@intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Acked-by: Manasi Navare <manasi.d.navare@intel.com>
Signed-off-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180117192149.17760-3-ville.syrjala@linux.intel.com