vga16fb: remove open_lock mutex

Remove mutex from the fb_open/fb_release functions as these operations are
mutexed at fb layer.

Signed-off-by: Krzysztof Helt <krzysztof.h1@wp.pl>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>