x264enc: Separate high-10 video formats from 8-bit formats

x264enc: Separate high-10 video formats from 8-bit formats

If downstream is constrained to an 8-bit profile, caps queries would
still allow I420_10LE as input. If upstream actually sends such a caps
event, downstream would fail to accept the high-10 profile.

The following pipeline now fails earlier, during the negotiation phase
instead of the stream start:

    gst-launch-1.0 videotestsrc ! video/x-raw,format=I420_10LE \
        ! x264enc ! video/x-h264,profile=constrained-baseline \
        ! fakesink

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-ugly/-/merge_requests/59>