tu: ir3: Emit push constants directly

tu: ir3: Emit push constants directly

Carve out some space at the beginning for push constants, and push them
directly, rather than remapping them to a UBO and then relying on the
UBO pushing code. Remapping to a UBO is easy now, where there's a single
table of UBO's, but with the bindless model it'll be a lot harder. I
haven't removed all the code to move the remaining UBO's over by 1,
though, because it's going to all get rewritten with bindless anyways.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4358>