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

Documentation/gpu/drm-kms.rst

@@ -33,7 +33,7 @@ Atomic Mode Setting Function Reference
 .. kernel-doc:: drivers/gpu/drm/drm_atomic.c
    :export:
 
-.. kernel-doc:: drivers/gpu/drm/drm_atomic.c
+.. kernel-doc:: include/drm/drm_atomic.h
    :internal:
 
 Frame Buffer Abstraction