arch-gcn3: make read2st64_b32 write proper registers

arch-gcn3: make read2st64_b32 write proper registers

Per the GCN3 ISA, read2st64_b32 writes to consecutive registers

Change-Id: Ibc1672584a72cf7de12e06068a03fe304b34dce2
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/32236
Reviewed-by: Matt Sinclair <mattdsinclair@gmail.com>
Reviewed-by: Alexandru Duțu <alexandru.dutu@amd.com>
Reviewed-by: Bradford Beckmann <brad.beckmann@amd.com>
Reviewed-by: Anthony Gutierrez <anthony.gutierrez@amd.com>
Maintainer: Anthony Gutierrez <anthony.gutierrez@amd.com>
Tested-by: kokoro <noreply+kokoro@google.com>