media: renesas: vsp1: Drop brx_get_compose() wrapper

The brx_get_compose() function is just a wrapper around
v4l2_subdev_state_get_compose() without any added value. Drop it and
call v4l2_subdev_state_get_compose() directly.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Jacopo Mondi <jacopo.mondi+renesas@ideasonboard.com>