drm/dp_mst: Don't cache EDIDs for physical ports

drm/dp_mst: Don't cache EDIDs for physical ports

Caching EDIDs for physical ports prevents updating the EDID if a port
gets reconnected via a Connection Status Notification message, fix this.

Fixes: db1a07956968 ("drm/dp_mst: Handle SST-only branch device case")
Cc: Wayne Lin <Wayne.Lin@amd.com>
Cc: Lyude Paul <lyude@redhat.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210201120145.350258-2-imre.deak@intel.com