KVM: arm64: Remove the wi->{e0,}poe vs wr->{p,u}ov confusion

KVM: arm64: Remove the wi->{e0,}poe vs wr->{p,u}ov confusion

Some of the POE computation is a bit confused. Specifically, there
is an element of confusion between what wi->{e0,}poe an wr->{p,u}ov
actually represent.

- wi->{e0,}poe is an *input* to the walk, and indicates whether
  POE is enabled at EL0 or EL{1,2}

- wr->{p,u}ov is a *result* of the walk, and indicates whether
  overlays are enabled. Crutially, it is possible to have POE
  enabled, and yet overlays disabled, while the converse isn't
  true

What this all means is that once the base permissions have been
established, checking for wi->{e0,}poe makes little sense, because
the truth about overlays resides in wr->{p,u}ov. So constructs
checking for (wi->poe && wr->pov) only add perplexity.

Refactor compute_s1_overlay_permissions() and the way it is
called according to the above principles. Take the opportunity
to avoid reading registers that are not strictly required.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20250701151648.754785-2-maz@kernel.org
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>