drm/dsi: Add routine to unregister a DSI device

A driver calling mipi_dsi_device_register_full might want to unregister
the device once it's done. It might also require it in an error handling
path in case something didn't go right.

Create mipi_dsi_device_unregister for this purpse, 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>