freedreno/ir3: Fix disasm of register offsets in ldp/stp.

freedreno/ir3: Fix disasm of register offsets in ldp/stp.

I had a stp testcase that was getting its offset wrong, and by twiddling
bits and feeding it to qc disasm, I found that the comment was sort of
right: some the cat6a bits implicated in the old comment do get used, as
the high bits of the cat6c offset.  Reallocating those bits also fixes how
we were getting r960.y for r0.y.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5815>