powerpc/pseries: Re-organise the oops compression code

nvram_compress() and zip_oops() is used by the nvram_pstore_write
API to compress oops messages hence re-organise the functions
accordingly to avoid forward declarations.

Signed-off-by: Aruna Balakrishnaiah <aruna@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>