drm/i915/bdw: Don't muck with gtt_size on Gen8 when PPGTT setup fails

v2: Resolve rebase conflicts and switch to gen < 8 color for GenX
checking.

v3: Rebase on top of the address space refactoring.

Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com> (v1)
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>