drm/atomic: add drm_atomic_get_existing_*_state helpers

There are cases where we want to test if a given object is
part of the state, but don't want to add them if they're not.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>