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>
diff --git a/Documentation/DocBook/drm.tmpl b/Documentation/DocBook/drm.tmpl
index efef637..ba09b6f 100644
--- a/Documentation/DocBook/drm.tmpl
+++ b/Documentation/DocBook/drm.tmpl
@@ -282,6 +282,16 @@
       </sect3>
     </sect2>
     <sect2>
+      <title>Device Registration</title>
+      <para>
+        A number of functions are provided to help with device registration.
+        The functions deal with PCI, USB and platform devices, respectively.
+      </para>
+!Edrivers/gpu/drm/drm_pci.c
+!Edrivers/gpu/drm/drm_usb.c
+!Edrivers/gpu/drm/drm_platform.c
+    </sect2>
+    <sect2>
       <title>Driver Load</title>
       <para>
         The <methodname>load</methodname> method is the driver and device