pxafb: convert ctrlr_sem in a mutex

The semaphore ctrlr_sem is used as a mutex.  Convert it to the mutex API.

Signed-off-by: Matthias Kaehlcke <matthias@kaehlcke.net>
Cc: Daniel Mack <daniel@caiaq.de>
Cc: Eric Miao <eric.miao@marvell.com>
Cc: Russell King <rmk@arm.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>