drm/i915/psr: Add connector debugfs files for MST connector as well

Connector debugfs files are currently not add for MST connector. We
can now add them as we have taken into account possibility to have
NULL in connector->encoder in intel_attached_dp.

v2: remove TODO comment

Signed-off-by: Jouni Högander <jouni.hogander@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240906070033.289015-4-jouni.hogander@intel.com