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