va: h264enc: Correct the hierarchical level name for pyramid-b frames.

va: h264enc: Correct the hierarchical level name for pyramid-b frames.

The current way names the level by the number of B frames it contains, the
less it contains, the higher level it is. So the non ref B frames are in the
lowest layer and the B frames in the highest level refer to I/P frames.
But the widely used way is just the opposite, the ref B frames are in the
lower level and non ref B frames are at the highest level.
The is just a terminology change, and does not have any effect for compression
result and quality.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/2149>