drm/tegra: dsi: Mark connector hotpluggable

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: Thierry Reding <treding@nvidia.com>