drm/i915: Zap mmaps of objects before unbinding them from the GTT.

Otherwise, some other userland writing into its buffer may race to land
writes either after the CPU thinks it's got a coherent view, or after its
GTT entries have been redirected to point at the scratch page.  Either
result is unpleasant.

Signed-off-by: Eric Anholt <eric@anholt.net>