WIP POSTING_READ fix series

[Eric Anholt <eric@anholt.net>]

Here's what I've been talking about on IRC today.  Patch 3 appears to
make things work.  Patch 4 should work, but re-breaks things (hangs
but no dmesg complaints).  I want to understand why, but I'm running
out of theories.  Perhaps there's some period of time where a write
has been PCI write posted but has not yet appeared in the GT FIFO?  Or
perhaps there's some time after GT FIFO but before it's really handled
by hardware?  Note that I was also able to fix the problem without
this series by just POSTING_READing 4 times instead of 1.