drm/i915/dp_mst: Cast intel_connector->port as drm_dp_mst_port

The only reason for having this cast as void * before was because we
originally needed to use drm_dp_mst_get_port_validated() and friends in
order to (attempt to) safely access MST ports. However, we've since
improved how reference counting works with ports and mstbs such that we
can now rely on drm_dp_mst_port structs remaining in memory for as long
as the driver needs. This means we don't really need to cast this as
void* anymore, and can just access the struct directly.

We'll also need this for the next commit, so that we can remove
drm_dp_mst_port_has_audio().

Signed-off-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Sean Paul <sean@poorly.run>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200406200646.1263435-1-lyude@redhat.com