drm/i915: disable stolen mem for OVERLAY_NEEDS_PHYSICAL

Our phys_object code can't deal with stolen memory and so blows up.
Fixing this is quite a bit of work and not worth it much for a single
page object, so just opt-out.

This is necessary prep work to enable stolen on gen2/3 platforms where
the overlay register file isn't stored in the gtt.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>