drm/i915: drop duplicate ggtt vma list add in setup_global_gtt

Preallocated objects will already have been added to the vma_list when
creating their ggtt vma entry, and coincidentally also marked as holding
a ggtt mapping. Repeating the vma_list manipulation when setting up the
ggtt after preallocation is a recipe for an unhappy kernel.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
[danvet: Use the improve commit message suggest by Chris.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>