drm/dsi: Add uevent callback

Implement a uevent callback for devices on the MIPI DSI bus. This
callback will append MODALIAS information to the uevent and allow
modules to be loaded when devices are added to the bus.

Reviewed-by: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>