drm/i915: Extend BIOS stolen mem handling to all platform

Based upon a patch from Deepak, but reworked to only apply on gen7+
and with the logic a bit clarified.

v2: Fix s/SHIFT/MASK/ fumble that Ville spotted.

Cc: Deepak S <deepak.s@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>