drm: cleanup: drm_modeset_lock_all_ctx() --> DRM_MODESET_LOCK_ALL_BEGIN()

As requested in Documentation/gpu/todo.rst, replace the boilerplate code
surrounding drm_modeset_lock_all_ctx() with DRM_MODESET_LOCK_ALL_BEGIN()
and DRM_MODESET_LOCK_ALL_END()

Signed-off-by: Fernando Ramos <greenfoo@u92.eu>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210924064324.229457-2-greenfoo@u92.eu