drm: rcar-du: Clarify vsp dependency

The VSP1 compositor code in DRM links against the respective V4L
driver, but the dependency is not expressed correctly in Kconfig,
which leads to a build error when the DRM driver is built-in
and the V4L driver is a module:

drivers/gpu/built-in.o: In function `rcar_du_vsp_plane_atomic_update':
rcar-du/rcar_du_vsp.c:183: undefined reference to `vsp1_du_atomic_update'

This patch avoids the problem by ensuring that the DRM VSP code can
only be enabled if the V4L driver is linked into the kernel, or
both are loadable modules.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: 6d62ef3 ("drm: rcar-du: Expose the VSP1 compositor through KMS planes")
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>

drivers/gpu/drm/rcar-du/Kconfig

@@ -27,6 +27,6 @@ config DRM_RCAR_LVDS
 config DRM_RCAR_VSP
 	bool "R-Car DU VSP Compositor Support"
 	depends on DRM_RCAR_DU
-	depends on VIDEO_RENESAS_VSP1
+	depends on VIDEO_RENESAS_VSP1=y || (VIDEO_RENESAS_VSP1 && DRM_RCAR_DU=m)
 	help
 	  Enable support to expose the R-Car VSP Compositor as KMS planes.