drm/i915: Drop vma is-closed assertion on insert

The is-closed flag may be added after we have acquired the vma under the
ctx->mutex, but will not take effect until after we release the
vm->mutex. i.e. the flag may be set on the vma as attempt to bind it and
that will cause the vma to be unbound later after we unpin it.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200303093157.1153887-1-chris@chris-wilson.co.uk