freedreno/a6xx: Decouple VFD_FETCH and VFD_DECODE

freedreno/a6xx: Decouple VFD_FETCH and VFD_DECODE

We used to output a VFD_FETCH entry for each VFD_DECODE, but we can
instead output just one VFD_FETCH per VBO and point multiple
VFD_DECODE entries at the same VFD_FETCH entry.  There's typically
fewer VBOs than vertex elements so this is a small win in itselfs, but
more importantly, the VFD_DECODE state now only depends on program
state.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5140>