video: fbdev: omapfb: Remove redundant 'flush_workqueue()' calls

'destroy_workqueue()' already drains the queue before destroying it, so
there is no need to flush it explicitly.

Remove the redundant 'flush_workqueue()' calls.

Signed-off-by: Xu Wang <vulab@iscas.ac.cn>
Signed-off-by: Helge Deller <deller@gmx.de>