drm/mipi-dbi: Add driver_private member to struct mipi_dbi_dev

devm_drm_dev_alloc() can't allocate structures that embed a structure
which then again embeds drm_device. Workaround this by adding a
driver_private pointer to struct mipi_dbi_dev which the driver can use for
its additional state.

v3:
- Add documentation
Acked-by: Maxime Ripard <maxime@cerno.tech>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20220227124713.39766-5-noralf@tronnes.org

include/drm/drm_mipi_dbi.h

@@ -130,6 +130,14 @@ struct mipi_dbi_dev {
 	 * @dbi: MIPI DBI interface
 	 */
 	struct mipi_dbi dbi;
+
+	/**
+	 * @driver_private: Driver private data.
+	 *                  Necessary for drivers with private data since devm_drm_dev_alloc()
+	 *                  can't allocate structures that embed a structure which then again
+	 *                  embeds drm_device.
+	 */
+	void *driver_private;
 };
 
 static inline struct mipi_dbi_dev *drm_to_mipi_dbi_dev(struct drm_device *drm)