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=97666Signed-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