vout: simplify flush locking

The static function vout_FlushUnlocked(), called from vout_Flush() and vout_ReleaseDisplay(), internally locked the display_lock.

From vout_Flush(), the display_lock is unnecessary because the function is called within a vout_control_Hold()/vout_control_Release() section.

From vout_ReleaseDisplay(), the display_lock was locked just after vout_FlushUnlocked(), so moving the call avoids to lock/unlock twice.