Instead of looking at encoder->type, which may be set to UNKNOWN,
use connector->connector_type. Info cannot be printed for MST
connectors which may have a NULL encoder, return early in that case.

Changes since v1:
- Whitelist encoder types for HDMI and LVDS.
- Fix oops on MST.
- Do not list encoder types for eDP/DP, they're always valid.
Signed-off-by: NMaarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: NVille Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/7cf34026-392d-01ec-e79b-e91919d1d783@linux.intel.com