drm/i915/display: Handle MST connector in intel_attached_dp

Connector->encoder might be null for MST connector. Take this into account
in intel_attached_dp.

Signed-off-by: Jouni Högander <jouni.hogander@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240906070033.289015-2-jouni.hogander@intel.com