xf86drmMode.h: make the subpixel enum actually match the kernel
The enum in the kernel starts at 0, not 1 (https://gitlab.com/linux-kernel/linux/-/blob/14f9643dc90adea074a0ffb7a17d337eafc6a5cc/include/drm/drm_connector.h#L137)
While this could be considered a breaking change, as this hasn't been noticed in 14 years of the enum being wrong, I think it should be fine?