reservation: cross-device reservation support, v4

This adds support for a generic reservations framework that can be
hooked up to ttm and dma-buf and allows easy sharing of reservations
across devices.

The idea is that a dma-buf and ttm object both will get a pointer
to a struct reservation_object, which has to be reserved before
anything is done with the contents of the dma-buf.

Changes since v1:
 - Fix locking issue in ticket_reserve, which could cause mutex_unlock
   to be called too many times.
Changes since v2:
 - All fence related calls and members have been taken out for now,
   what's left is the bare minimum to be useful for ttm locking conversion.
Changes since v3:
 - Removed helper functions too. The documentation has an example
   implementation for locking. With the move to ww_mutex there is no
   need to have much logic any more.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@canonical.com>
Reviewed-by: Jerome Glisse <jglisse@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>