video: sii9234: dynamically register and unregister input device

The RCP input device should only be registered as long as MHL is
connected

Original author for tuna: Greg Hackmann <ghackmann@google.com>

Fixes: http://jira.omnirom.org/browse/OMNI-685

Signed-off-by: Andreas B <skate4life@gmx.de>
Change-Id: Iddf527861f06e7b7b042f22e86f0a0ae410b6c12