drm/doc: render drm.h uapi docs

It doesn't seem like drm.h docs are included anywhere. Render them next
to drm_mode.h, under the "Userspace API Structures" section.

This also allows references to e.g. DRM_CAP_* to be properly linkified
elsewhere in our docs.
Signed-off-by: Simon Ser <contact@emersion.fr>
Acked-by: Daniel Vetter <daniel@ffwll.ch>
Cc: Pekka Paalanen <ppaalanen@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201222133524.160842-6-contact@emersion.fr

Documentation/gpu/drm-uapi.rst

@@ -457,5 +457,8 @@ Userspace API Structures
 .. kernel-doc:: include/uapi/drm/drm_mode.h
    :doc: overview
 
+.. kernel-doc:: include/uapi/drm/drm.h
+   :internal:
+
 .. kernel-doc:: include/uapi/drm/drm_mode.h
    :internal: