drm: Make drm_atomic_helper.h standalone includible

This header uses a bunch of declarations from the drm/drm_crtc.h header,
so make sure to include that as well so that drm_atomic_helper.h can be
included standalone.
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

include/drm/drm_atomic_helper.h

@@ -28,6 +28,8 @@
 #ifndef DRM_ATOMIC_HELPER_H_
 #define DRM_ATOMIC_HELPER_H_
 
+#include <drm/drm_crtc.h>
+
 int drm_atomic_helper_check(struct drm_device *dev,
 			    struct drm_atomic_state *state);
 int drm_atomic_helper_commit(struct drm_device *dev,