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: 6d62ef3a ("drm: rcar-du: Expose the VSP1 compositor through KMS planes")
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>