target/ppc: Don't store VRMA SLBE persistently

target/ppc: Don't store VRMA SLBE persistently

Currently, we construct the SLBE used for VRMA translations when the LPCR
is written (which controls some bits in the SLBE), then use it later for
translations.

This is a bit complex and confusing - simplify it by simply constructing
the SLBE directly from the LPCR when we need it.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>