x86: change aper valid checking sequence

old sequence:
  size ==> >4G  ==> point to RAM

changed to:
  >4G ==> point to RAM ==> size

some bios even leave aper to unclear, so check size at last.

To avoid reporting:

  Node 0: Aperture @ 4a42000000 size 32 MB
  Aperture too small (32 MB)

with this change we will get:

  Node 0: Aperture @ 4a42000000 size 32 MB
  Aperture beyond 4G. Ignoring.

Signed-off-by: Yinghai Lu <yinghai.lu@sun.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>