DSI panels can always be hotplugged via the DSI bus' attach/detach
infrastructure, so unconditionally mark the connector hotpluggable.

While at it, also make sure that when a panel is detached the connector
is marked unconnected before calling into the DRM hotplug helpers to
reflect the correct state.
Signed-off-by: NThierry Reding <treding@nvidia.com>