drm/i915: Verify that our stolen memory doesn't conflict

Sanity check that the memory region found through the Graphics Base
of Stolen Memory is reserved and hidden from the rest of the system
through the use of the resource API.

v2: "Graphics Stolen Memory" is such a more bodacious name than the lame
    "i915 stolen", and convert to using devres for automagical cleanup of
    the resource. (danvet)

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
[danvet: Dump proper hexcodes.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>