drm/fb-helper: Add generic fbdev emulation .fb_probe function

This is the first step in getting generic fbdev emulation.
A drm_fb_helper_funcs.fb_probe function is added which uses the
DRM client API to get a framebuffer backed by a dumb buffer.

Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20180703160354.59955-3-noralf@tronnes.org
diff --git a/include/drm/drm_fb_helper.h b/include/drm/drm_fb_helper.h
index b069433..c134bbc 100644
--- a/include/drm/drm_fb_helper.h
+++ b/include/drm/drm_fb_helper.h
@@ -32,6 +32,7 @@
 
 struct drm_fb_helper;
 
+#include <drm/drm_client.h>
 #include <drm/drm_crtc.h>
 #include <drm/drm_device.h>
 #include <linux/kgdb.h>
@@ -154,6 +155,20 @@ struct drm_fb_helper_connector {
  * operations.
  */
 struct drm_fb_helper {
+	/**
+	 * @client:
+	 *
+	 * DRM client used by the generic fbdev emulation.
+	 */
+	struct drm_client_dev client;
+
+	/**
+	 * @buffer:
+	 *
+	 * Framebuffer used by the generic fbdev emulation.
+	 */
+	struct drm_client_buffer *buffer;
+
 	struct drm_framebuffer *fb;
 	struct drm_device *dev;
 	int crtc_count;
@@ -234,6 +249,12 @@ struct drm_fb_helper {
 	int preferred_bpp;
 };
 
+static inline struct drm_fb_helper *
+drm_fb_helper_from_client(struct drm_client_dev *client)
+{
+	return container_of(client, struct drm_fb_helper, client);
+}
+
 /**
  * define DRM_FB_HELPER_DEFAULT_OPS - helper define for drm drivers
  *
@@ -330,6 +351,9 @@ void drm_fb_helper_fbdev_teardown(struct drm_device *dev);
 
 void drm_fb_helper_lastclose(struct drm_device *dev);
 void drm_fb_helper_output_poll_changed(struct drm_device *dev);
+
+int drm_fb_helper_generic_probe(struct drm_fb_helper *fb_helper,
+				struct drm_fb_helper_surface_size *sizes);
 #else
 static inline void drm_fb_helper_prepare(struct drm_device *dev,
 					struct drm_fb_helper *helper,
@@ -564,6 +588,13 @@ static inline void drm_fb_helper_output_poll_changed(struct drm_device *dev)
 {
 }
 
+static inline int
+drm_fb_helper_generic_probe(struct drm_fb_helper *fb_helper,
+			    struct drm_fb_helper_surface_size *sizes)
+{
+	return 0;
+}
+
 #endif
 
 static inline int