target/i386: Fix test for paging enabled

If CR0.PG is unset, pg_mode will be zero, but it will also be zero
for non-PAE/non-PSE page tables with CR0.WP=0.  Restore the
correct test for paging enabled.

Fixes: 98281984 ("target/i386: Add MMU_PHYS_IDX and MMU_NESTED_IDX")
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1269


Reported-by: Andreas Gustafsson <gson@gson.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20221102091232.1092552-1-richard.henderson@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>