powerpc: fix boot on pseries without hugepages

Commit 49c894ddced5 introduced an issue that prevented pseries boot,
when hugepages were not available to the guest. Now large page
info must be available before moea64_install is called, so this change
moves the code that scans large page sizes before the call.

Reviewed by: jhibbits (IRC)
Sponsored by: Instituto de Pesquisas Eldorado (eldorado.org.br)