media: tegra_camera: replace workqueue with kthread

media: tegra_camera: replace workqueue with kthread

Use kthread instead of workqueue, which will create a dedicated kernel
thread for capture.

Remove useless mutex and convert spin_lock_irq() to normal spin_lock().

Bug 1686911

Change-Id: Ie90320ccff9b46a53719bf8924afe38b24dda64c
Signed-off-by: Bryan Wu <pengw@nvidia.com>
Reviewed-on: http://git-master/r/824953
(cherry picked from commit d184f7a5ad79419a169c01e453af2c5ac41b5865)
Reviewed-on: http://git-master/r/824964
Reviewed-by: Matthew Pedro <mapedro@nvidia.com>