drm/i915: Set stolen reserved to 0 for pre-g4x platforms

drm/i915: Set stolen reserved to 0 for pre-g4x platforms

This stolen reserved stuff was introduced on g4x, so no need to waste
stolen on older platforms. Unfortunately configdb is no more so I can't
look up the right way to detect this stuff. I do have one hint as to
where the register might be on ctg, but I don't have a ctg to test it,
and on the elk I have here it doesn't contain sensible looking data.
For ilk grits suggegsts it might be in the same place as on snb (the
original PCI reg, not the mirror) but I can't be entirely sure about it
The register shows a round zero on my ilk.

So when there's no really good data for any of these platforms leave the
current "assume 1MiB" approach in place.

Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Acked-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>