Stolen gets trashed during hibernation, so storing contexts there
is not a very good idea. On my IVB machines this leads to a totally
dead GPU on resume. A reboot is required to resurrect it. So let's
not store contexts where they will get trampled.

This reverts commit 149c86e7.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Acked-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>