drm_atomic_state_put doesn't require any locking, and this makes things
easier for switching to modeset_lock_all helpers in a future patch

Changes in v2:
- Moved state->acquire_ctx clear to a separate patch (Daniel)
Reviewed-by: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20181129150423.239081-3-sean@poorly.run