drm/dp/mst: Clear port->pdt when tearing down the i2c adapter

The i2c adapter is only relevant for some peer device types, so
let's clear the pdt if it's still the same as the old_pdt when we
tear down the i2c adapter.

I don't really like this design pattern of updating port->whatever
before doing the accompanying changes and passing around old_whatever
to figure stuff out. Would make much more sense to me to the pass the
new value around and only update the port->whatever when things are
consistent. But let's try to work with what we have right now.

Quoting a follow-up from Ville:

"And naturally I forgot to amend the commit message w.r.t. this guy
[the change in drm_dp_destroy_port].  We don't really need to do this
here, but I figured I'd try to be a bit more consistent by having it,
just to avoid accidental mistakes if/when someone changes this stuff
again later."

v2: Clear port->pdt in the caller, if needed (Daniel)

Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: stable@vger.kernel.org
Cc: Carlos Santa <carlos.santa@intel.com>
Cc: Kirill A. Shutemov <kirill@shutemov.name>
Tested-by: Carlos Santa <carlos.santa@intel.com> (v1)
Tested-by: Kirill A. Shutemov <kirill@shutemov.name> (v1)
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=97666
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1477488633-16544-1-git-send-email-ville.syrjala@linux.intel.com