x86/pat: Restructure _lookup_address_cpa()

Modify _lookup_address_cpa() to no longer use lookup_address(), but
only lookup_address_in_pgd().

This is done in preparation of using lookup_address_in_pgd_attr().

No functional change intended.

Signed-off-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20240412151258.9171-4-jgross@suse.com