drm/radeon/kms: record object that have been list reserved

list reservation was too optimistic about ttm object reservation
and could think that an object reserved by some other process
as reserved by the list reservation which was false. Thus when
unreserving the list it might unreserve object that it didn't
reserved in the list. Sorry if it's hard to follow but this
kind of things are just causing headheck.

Signed-off-by: Jerome Glisse <jglisse@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>