drm/dp/mst: Check peer device type before attempting EDID read

drm/dp/mst: Check peer device type before attempting EDID read

commit 4da5caa6a6f82cda3193bca855235b87debf78bd upstream.

Only certain types of pdts have the DDC bus registered, so check for
that before we attempt the EDID read. Othwewise we risk playing around
with an i2c adapter that doesn't actually exist.

Cc: Carlos Santa <carlos.santa@intel.com>
Cc: Kirill A. Shutemov <kirill@shutemov.name>
Tested-by: Carlos Santa <carlos.santa@intel.com>
Tested-by: Kirill A. Shutemov <kirill@shutemov.name>
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/1477472755-15288-5-git-send-email-ville.syrjala@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>