drm/atomic: use connector references (v3)

Take a reference when setting a crtc on a connecter,
also take one when duplicating if a crtc is set,
and drop one on destroy if a crtc is set.

v2: take Daniel Stone's advice and simplify the
ref/unref dances, also take care of NULL as connector
to state reset.

v3: remove need for connector NULL check.

Reviewed-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>