drm/atomic: integrate modeset lock with private objects

Follow the same pattern of locking as with other state objects. This
avoids boilerplate in the driver.

Signed-off-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20181022123122.30468-1-boris.brezillon@bootlin.com