drm/i915,agp/intel: Do not clear stolen entries

We can only utilize the stolen portion of the GTT if we are in sole
charge of the hardware. This is only true if using GEM and KMS,
otherwise VESA continues to access stolen memory.

Reported-by: Arnd Bergmann <arnd@arndb.de>
Reported-by: Frederic Weisbecker <fweisbec@gmail.com>
Tested-by: Jiri Olsa <jolsa@redhat.com>
Tested-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>