get_colorspace() checks input caps transfer when mapping V4L2_XFER_FUNC_709 back to V4L2_COLORSPACE_BT2020 and GST_VIDEO_TRANSFER_BT2020_12. After receiving source change event, decoder will G_FMT and S_FMT again. So need to reset transfer when acquiring format to avoid using the old transfer. Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/2475>