drm/atomic: Add __must_check to drm_atomic_helper_swap_state.

drm/atomic: Add __must_check to drm_atomic_helper_swap_state.

Now that all drivers check the return value, convert swap_state to
__must_check. This is done separately to force build warnings if we
missed a driver.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170711143314.2148-12-maarten.lankhorst@linux.intel.com
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>