drm/i915: Shove the PHY test into the hotplug work

drm/i915: Shove the PHY test into the hotplug work

Doing any kind modeset stuff from the short hpd handler is
verboten. The ad-hoc PHY test modeset code violates this. And
by calling various link training related functions it's now
blocking further work to plumb the crtc state down into the
link training code.

Let's hack around that by pushing the PHY test stuff into the
hotplug work where it's less of a problem. Still not great but
at least acceptable. We take a few pages from the link retraining
handbook to handle the locking and whatnot.

v2: Fix the intel_dp_hotplug() return value

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200930100412.9313-1-ville.syrjala@linux.intel.com
Reviewed-by: Imre Deak <imre.deak@intel.com>