etnaviv: implement UBOs
At the same time, use pre-HALTI2 to use address register for indirect uniform loads, since integers/LOAD instruction isn't always available.
Passes all dEQP-GLES3.functional.ubo.*
on GC7000L (needs ETNA_MESA_DEBUG=nir,deqp). GC3000 with an extra
flush hack passes most of them, but still fails on some of the cases with
many loads.
/cc @austriancoder @marex