drm: rcar-du: Fix Kconfig dependency between RCAR_DU and RCAR_MIPI_DSI

When the R-Car MIPI DSI driver was added, it was a standalone encoder
driver without any dependency to or from the R-Car DU driver. Commit
957fe62d ("drm: rcar-du: Fix DSI enable & disable sequence") then
added a direct call from the DU driver to the MIPI DSI driver, without
updating Kconfig to take the new dependency into account. Fix it the
same way that the LVDS encoder is handled.

Fixes: 957fe62d ("drm: rcar-du: Fix DSI enable & disable sequence")
Reported-by: kernel test robot <lkp@intel.com>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>

drivers/gpu/drm/rcar-du/Kconfig

@@ -44,13 +44,18 @@ config DRM_RCAR_LVDS
 	select OF_FLATTREE
 	select OF_OVERLAY
 
-config DRM_RCAR_MIPI_DSI
-	tristate "R-Car DU MIPI DSI Encoder Support"
-	depends on DRM && DRM_BRIDGE && OF
-	select DRM_MIPI_DSI
+config DRM_RCAR_USE_MIPI_DSI
+	bool "R-Car DU MIPI DSI Encoder Support"
+	depends on DRM_BRIDGE && OF
+	default DRM_RCAR_DU
 	help
 	  Enable support for the R-Car Display Unit embedded MIPI DSI encoders.
 
+config DRM_RCAR_MIPI_DSI
+	def_tristate DRM_RCAR_DU
+	depends on DRM_RCAR_USE_MIPI_DSI
+	select DRM_MIPI_DSI
+
 config DRM_RCAR_VSP
 	bool "R-Car DU VSP Compositor Support" if ARM
 	default y if ARM64