The VP9 related definitions in mfxvp9.h are available under the
condition of 'MFX_VERSION >= MFX_VERSION_NEXT', which implies that these
definitions are never used in a public release.
This is in preparation for oneVPL support because mfxvp9.h was
removed from oneVPL
Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-bad/-/merge_requests/1503>