drm/i915: Use Graphics Base of Stolen Memory on all gen3+

So I made the mistake of missing that the desktop and mobile chipsets
have different layouts in their PCI configurations, and we were
incorrectly setting the wrong physical address for stolen memory on
mobile chipsets.

Since all gen3+ are actually consistent in the location of the GBSM
register in the PCI configuration space on device 2 (the GPU), use it.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
[danvet: Drop cc: stable and fudge conflicts.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>