OMAP: DSS2: fix lock_fb_info() and omapfb_lock() locking order

Framebuffer ioctl processing forces lock_fb_info() -> omapfb_lock()
locking order. Follow that order to avoid possible circular locking
dependency, detected by lockdep.

Signed-off-by: Jani Nikula <ext-jani.1.nikula@nokia.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@nokia.com>