drm: rcar-du: Fix Kconfig dependency between DRM and RZG2L_MIPI_DSI

When CONFIG_DRM=m and CONFIG_DRM_RZG2L_MIPI_DSI=y, it results in a
build failure. This patch fixes the build issue by adding dependency
to DRM.

Fixes: 7a043f97 ("drm: rcar-du: Add RZ/G2L DSI driver")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>

drivers/gpu/drm/rcar-du/Kconfig

@@ -51,7 +51,7 @@ config DRM_RCAR_MIPI_DSI
 
 config DRM_RZG2L_MIPI_DSI
 	tristate "RZ/G2L MIPI DSI Encoder Support"
-	depends on DRM_BRIDGE && OF
+	depends on DRM && DRM_BRIDGE && OF
 	depends on ARCH_RENESAS || COMPILE_TEST
 	select DRM_MIPI_DSI
 	help