Previous to the last GTT rework we always rewrote the GTT then unmapped the
object, somehow this got reversed in the rework in 2.6.37-rc5 timeframe.

This fix needs to go to stable in an alternate form since the code changed.

This fixes DMAR reports on my Ironlake HP2540p.
Signed-off-by: Dave Airlie <airlied@redhat.com>