drm/i915: Introduce i915_gem_object_create_stolen_for_preallocated

Wrap a preallocated region of stolen memory within an ordinary GEM
object, for example the BIOS framebuffer.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>