drm: simple_kms_helper: Add mode_valid() callback support

The PL111 needs to filter valid modes based on memory bandwidth.
I guess it is a pretty simple operation, so we can still claim
the DRM KMS helper pipeline is simple after adding this (optional)
vtable callback.

Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20180220072859.3386-1-linus.walleij@linaro.org

include/drm/drm_simple_kms_helper.h

@@ -21,6 +21,20 @@ struct drm_simple_display_pipe;
  *                                        display pipeline
  */
 struct drm_simple_display_pipe_funcs {
+	/**
+	 * @mode_valid:
+	 *
+	 * This function is called to filter out valid modes from the
+	 * suggestions suggested by the bridge or display. This optional
+	 * hook is passed in when initializing the pipeline.
+	 *
+	 * RETURNS:
+	 *
+	 * drm_mode_status Enum
+	 */
+	enum drm_mode_status (*mode_valid)(struct drm_crtc *crtc,
+					   const struct drm_display_mode *mode);
+
 	/**
 	 * @enable:
 	 *