drm: rcar-du: Fix build error

Select DRM_KMS_HELPER dependency.

Build error when DRM_KMS_HELPER is not selected:

drivers/gpu/drm/rcar-du/rcar_lvds.o:(.rodata+0xd48): undefined reference to `drm_atomic_helper_bridge_duplicate_state'
drivers/gpu/drm/rcar-du/rcar_lvds.o:(.rodata+0xd50): undefined reference to `drm_atomic_helper_bridge_destroy_state'
drivers/gpu/drm/rcar-du/rcar_lvds.o:(.rodata+0xd70): undefined reference to `drm_atomic_helper_bridge_reset'
drivers/gpu/drm/rcar-du/rcar_lvds.o:(.rodata+0xdc8): undefined reference to `drm_atomic_helper_connector_reset'
drivers/gpu/drm/rcar-du/rcar_lvds.o:(.rodata+0xde0): undefined reference to `drm_helper_probe_single_connector_modes'
drivers/gpu/drm/rcar-du/rcar_lvds.o:(.rodata+0xe08): undefined reference to `drm_atomic_helper_connector_duplicate_state'
drivers/gpu/drm/rcar-du/rcar_lvds.o:(.rodata+0xe10): undefined reference to `drm_atomic_helper_connector_destroy_state'

Fixes: c6a27fa4 ("drm: rcar-du: Convert LVDS encoder code to bridge driver")
Cc: <stable@vger.kernel.org>
Signed-off-by: Daniel Gomez <dagmcr@gmail.com>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>