drm: rcar-du: Depend on DRM_RCAR_DU for components on that SoC

There's a few components in the rcar-du drm directory that don't make
sense to be selectable if DRM_RCAR_DU isn't because they are part of the
IP block so add a dependency. The drivers are still testable with
COMPILE_TEST on non-Renesas platforms as DRM_RCAR_DU itself is
selectable with COMPILE_TEST.
Signed-off-by: Peter Robinson <pbrobinson@gmail.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>

drivers/gpu/drm/rcar-du/Kconfig

@@ -25,6 +25,7 @@ config DRM_RCAR_CMM
 config DRM_RCAR_DW_HDMI
 	tristate "R-Car Gen3 and RZ/G2 DU HDMI Encoder Support"
 	depends on DRM && OF
+	depends on DRM_RCAR_DU || COMPILE_TEST
 	select DRM_DW_HDMI
 	help
 	  Enable support for R-Car Gen3 or RZ/G2 internal HDMI encoder.
@@ -32,6 +33,7 @@ config DRM_RCAR_DW_HDMI
 config DRM_RCAR_USE_LVDS
 	bool "R-Car DU LVDS Encoder Support"
 	depends on DRM_BRIDGE && OF
+	depends on DRM_RCAR_DU || COMPILE_TEST
 	default DRM_RCAR_DU
 	help
 	  Enable support for the R-Car Display Unit embedded LVDS encoders.
@@ -45,6 +47,7 @@ config DRM_RCAR_LVDS
 config DRM_RCAR_USE_MIPI_DSI
 	bool "R-Car DU MIPI DSI Encoder Support"
 	depends on DRM_BRIDGE && OF
+	depends on DRM_RCAR_DU || COMPILE_TEST
 	default DRM_RCAR_DU
 	help
 	  Enable support for the R-Car Display Unit embedded MIPI DSI encoders.