drm/i915: Enforce write ordering through the GTT

We need to ensure that writes through the GTT land before any
modification to the MMIO registers and so must impose a mandatory write
barrier when flushing the GTT domain. This was revealed by relaxing the
write ordering by experimentally mapping the registers and the GATT as
write-combining.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>