drm/doc: Include drm_atomic.h

Accidentally the wrong file. Oops.

Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1471034937-651-10-git-send-email-daniel.vetter@ffwll.ch