drm/i915: Fix broken mst get_hw_state.

connector->encoder is initialized as NULL. Fix this by setting it in
during pre enable. MST connectors are not read out during initial hw
readout, and have no fixed encoder mappings. So it's harmless to
return false when the connector has never been assigned to an encoder.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Ander Conselvan de Oliveira <conselvan2@gmail.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>