drm: rcar-du: Replace DRM_INFO() with drm_info()

drm_info() adds proper context to the kernel log message, as it receives
the drm_device pointer. It is thus preferred over DRM_INFO(). Replace
the latter with the former.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>