drm/i915: use the ICL stolen memory

Now that our stolen memory is already reserved by the x86 subsystem
(since commit "x86/gpu: reserve ICL's graphics stolen memory"), make
use of it.

Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: x86@kernel.org
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180504203252.28048-2-paulo.r.zanoni@intel.com