drm/i915: nuke the duplicated stolen discovery

We duplicate the stolen discovery code in early-quirks and in i915,
however now that the stolen region is exported as a resource from
early-quirks we can nuke the duplication.

v2: check overflows_type

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171211151822.20953-5-matthew.auld@intel.com