adv7481: Changes to compile ADV7481 as a module

ADV7481 driver is board-specific, and thus should be compiled
as a module. This is part of the kernel modularization requirement.

Change-Id: I38ab9efca34ac9f898c2b76843563d8f74c29320
Suggested-by: Resmi Rajendran <resmir@qti.qualcomm.com>
Signed-off-by: Rahul Sharma <sharah@codeaurora.org>