x86: move pgd_base out of init_memory_mapping()

Impact: cleanup

This patch moves pgd_base out of init_memory_mapping() to reduce
the diff between the 32-bit version and the 64-bit version of the
function.

Signed-off-by: Pekka Enberg <penberg@cs.helsinki.fi>
Cc: Yinghai Lu <yinghai@kernel.org>
LKML-Reference: <1236257708-27269-4-git-send-email-penberg@cs.helsinki.fi>
Signed-off-by: Ingo Molnar <mingo@elte.hu>