nvidiafb: remove open_lock mutex

Remove mutex from the nvidiafb_open/nvidiafb_release functions as these
operations are mutexed at fb layer.

Signed-off-by: Alessio Igor Bogani <abogani@texware.it>
Cc: Krzysztof Helt <krzysztof.h1@wp.pl>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>