drm/modeset-helper: Add simple modeset suspend/resume helpers

Add drm_mode_config_helper_suspend/resume() which takes care of
atomic modeset suspend/resume for simple use cases.
The suspend state is stored in struct drm_mode_config.

Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20171106191812.38927-3-noralf@tronnes.org

include/drm/drm_mode_config.h

@@ -753,6 +753,15 @@ struct drm_mode_config {
 	/* cursor size */
 	uint32_t cursor_width, cursor_height;
 
+	/**
+	 * @suspend_state:
+	 *
+	 * Atomic state when suspended.
+	 * Set by drm_mode_config_helper_suspend() and cleared by
+	 * drm_mode_config_helper_resume().
+	 */
+	struct drm_atomic_state *suspend_state;
+
 	const struct drm_mode_config_helper_funcs *helper_private;
 };