OMAP: DSS2: DSI: change DSI bus_lock to semaphore

Physical DSI bus is protected by a mutex. This patch changed the mutex to
a semaphore, so that we can lock and unlock the bus_lock from different
threads.

This is needed as the update process is started by user space program, and
thus the lock is acquired in that context, but the lock can be released in
different context, a work thread via irq.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@nokia.com>