drm: ADD UAPI structure definition section in kernel doc

Add a new section for UAPI structure and helper definitions
in kernel docbook.
Suggested-by: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Uma Shankar <uma.shankar@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/1559567330-25182-2-git-send-email-uma.shankar@intel.com

Documentation/gpu/drm-uapi.rst

@@ -329,3 +329,12 @@ DRM_IOCTL_MODESET_CTL
     mode setting, since on many devices the vertical blank counter is
     reset to 0 at some point during modeset. Modern drivers should not
     call this any more since with kernel mode setting it is a no-op.
+
+Userspace API Structures
+========================
+
+.. kernel-doc:: include/uapi/drm/drm_mode.h
+   :doc: overview
+
+.. kernel-doc:: include/uapi/drm/drm_mode.h
+   :internal: