MLK-19673-2: drm/imx/dcss: protect against concurent commits

MLK-19673-2: drm/imx/dcss: protect against concurent commits

The current DCSS driver uses the generic drm_atomic_helper_commit().
But, this helper offers no protection against concurent commits by
userspace apps that may not wait for flip_done events.

This patch customizes the atomic_commit() callback by reusing the
drm_atomic_helper_commit() helper and adding a spinlock that will not
allow for another commit to go through if one is already pending.

Since we'll be calling the dcss_drm_atomic_commit_tail() ourselves,
there's no need for drm_mode_config_helper_funcs anymore. So, remove it.

Signed-off-by: Laurentiu Palcu <laurentiu.palcu@nxp.com>