freedreno/ir3: Setup ir3 inputs and outputs for GS

freedreno/ir3: Setup ir3 inputs and outputs for GS

Inputs are the GS header, which contains vertex ID, local primitive ID
and thread ID as well as primitive ID. The setup is a little different
from other sysvals, since we always have to receive them in the VS so
that it can pass them on into the GS.

The vertex flag outputs from GS is set up as a proper nir output in
the lowering pass and doesn't need special handling here.

Signed-off-by: Kristian H. Kristensen <hoegsberg@google.com>