drm/doc: Refer to proper source file

Commit 21d70354

 ("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 <treding@nvidia.com>
Reviewed-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>