drm/i915/debugfs: Do not print cached information of a disconnected sink

Besides of give the expected output of i915_display_info it will also
avoid some aux ch transactions that would timeout by obvious reasons.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181030215750.28213-1-jose.souza@intel.com