drm/i915: Place the Global GTT VM first in the list of VM

This is useful for debugging as we then know that the first entry is
always the global GTT, and all later entries the per-process GTT VM.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>