rm/i915: Move i915_get_ggtt_vma_pages into ggtt_bind_vma

We have this neat abstraction between ppgtt and ggtt for (un)bind_vma
and didn't end up using it really. What a shame, so fix this and make
the ->bind_vma hook a bit more useful.

Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>