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