drm/i915: Sanity check the computed size and base of stolen memory

Just do a quick check that the stolen memory address range doesn't
overflow our chosen integer type.

v2: Add add_overflows() to utils with the promise that gcc7 can do this
better than C and then maybe it will have a proper definition in core.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170130134721.5159-1-chris@chris-wilson.co.uk
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>