drm/i915: Handle i915_ppgtt_put correctly

Unfortunately, the gem_obj/vma relationship is not symmetrical; a gem_obj
can look up for the same vma more than once (where the ppgtt refcount is
incremented), but will free the vma only once (i915_gem_free_object).

This difference in refcount get/put means that the ppgtt is not removed
after the context and vma are destroyed, because sometimes the refcount
will never go back to zero.

v2: Just move the ppgtt refcount into vma_create.

OTC-Jira: VIZ-3719
Signed-off-by: Michel Thierry <michel.thierry@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>