drm/dsi: Add routine to unregister a DSI device

A driver calling mipi_dsi_device_register_full() might want to remove
the device once it's done. It might also require it in an error handling
path in case something went wrong.

Create mipi_dsi_device_unregister() for this purpose and use it within
mipi_dsi_remove_device_fn() as it does the same thing.

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