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>