video/msm_fb: use system_wq instead of dedicated workqueues

With cmwq, there's no reason to use separate workqueues.  Drop
msmfb_info->resume_workqueue and use system_wq instead.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Stanislaw Gruszka <stf_xl@wp.pl>
Cc: linux-usb@vger.kernel.org
Signed-off-by: David Brown <davidb@codeaurora.org>