arm64/vmm: Enable 16-bit VMIDs when in use by pmap

pmap_init always uses 16-bit VMIDs when supported, but we never enable
them in VTCR_EL2 (for ASIDs, locore enables them in TCR_EL1 and
pmap_init keys off whether they've been enabled, but the order in which
pmap_init and vmmops_modinit run is reversed). As a result, although the
full 16-bit value can be stored to VTTBR_EL2 and read back, the upper 8
bits are treated as 0, and so VMIDs that our VMID allocation believes
are distinct end up aliasing.

In future this interface may change such that vmm decides on the VMID
width and tells the pmap to use that, with appropriate support for
unloading and reloading vmm, but that can come as a follow-up change, as
this is a more minimal bug fix.

Reviewed by: markj
Obtained from: CheriBSD
Fixes: 47e073941f4e ("Import the kernel parts of bhyve/arm64")
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D55860

(cherry picked from commit 44e4f45aec4469440af434098e61bd13982bbbc0)