drm: Add device registration documentation

Describe how devices are registered using the drm_*_init() functions.
Adding this to docbook requires a largish set of changes to the comments
in drm_{pci,usb,platform}.c since they are doxygen-style rather than
proper kernel-doc and therefore mess with the docbook generation.

While at it, mark usage of drm_put_dev() as discouraged in favour of
calling drm_dev_unregister() and drm_dev_unref() directly.

Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Thierry Reding <treding@nvidia.com>