Since the commit af7a06bac7d3abb2da48ef3277d2a415772d2ae8:
`casa [..](10), .., ..` (and probably others alternate space instructions)
triggers a data access exception when the MMU is disabled.
When we enter get_asi(...) dc->mem_idx is set to MMU_PHYS_IDX when the MMU
is disabled. Just keep mem_idx unchanged in this case so we passthrough the
MMU when it is disabled.
Backports commit 6e10f37c86068e35151f982c976a85f1bec07ef2 from qemu