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>