gallium: reorder the shader stage enum to match Mesa
This has been a source of annoyance as we've moved mesa and gallium closer. Reordering them to be the same removes a source of bugs, but also means we don't have to think about it any more.
Fallout found so far:
virgl: wire encoding
radeonsi: bases a size of an array
zink: encoders the enum bit fields into a templated cache lookup
svga: uses TESS_EVAL as last graphics shader.
Otherwise this seems to pass CI.