drm: Make drm_atomic.h standalone includible

This header file makes use of a bunch of structures declared in the
drm_crtc.h header file. Include that to make sure the drm_atomic.h
header can be included standalone.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>