drm/i915: Remove vma from object on destroy, not close

Originally we translated from the object to the vma by walking
obj->vma_list to find the matching vm (for user lookups). Now we process
user lookups using the rbtree, and we only use obj->vma_list itself for
maintaining state (e.g. ensuring that all vma are flushed or rebound).
As such maintenance needs to go on beyond the user's awareness of the
vma, defer removal of the vma from the obj->vma_list from i915_vma_close()
to i915_vma_destroy()

Fixes: 5888fc9e

 ("drm/i915: Flush pending GTT writes before unbinding")
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=104155
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171206124914.19960-1-chris@chris-wilson.co.uk
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>