Implement initial atomic state handling. Hook up the CRTCs, planes' and
connectors' ->atomic_destroy_state() callback to ensure that the atomic
state objects don't leak.

Furthermore the CRTC now implements the ->mode_set_nofb() callback that
is used by new helpers to implement ->mode_set() and ->mode_set_base().
These new helpers also make use of the new plane helper functions which
the driver now provides.
Signed-off-by: Thierry Reding <treding@nvidia.com>