drm/doc: Refer to proper source file

Commit 21d70354bba9 ("drm: move drm_stub.c to drm_drv.c") moves the code
from drm_stub.c into drm_drv.c. Update DocBook to include that instead.

This also came in via other people, but all the same.

Signed-off-by: Thierry Reding <[e-mail]>
Reviewed-by: David Herrmann <[e-mail]>
Signed-off-by: Dave Airlie <[e-mail]>

Documentation/DocBook/drm.tmpl

···
         <function>drm_dev_unregister()</function> followed by a call to
         <function>drm_dev_unref()</function>.
       </para>
-!Edrivers/gpu/drm/drm_stub.c
+!Edrivers/gpu/drm/drm_drv.c
     </sect2>
     <sect2>
       <title>Driver Load</title>