docs-rst: fix LaTeX \DURole renewcommand with Sphinx 1.3+

PDF build on Kernel 4.9-rc? returns an error with Sphinx 1.3.x
and Sphinx 1.4.x, when trying to solve some cross-references.

The solution is to redefine the \DURole macro.

However, this is redefined too late. Move such redefinition to
LaTeX preamble and bind it to just the Sphinx versions where the
error is known to be present.

Tested by building the documentation on interactive mode:
	make PDFLATEX=xelatex -C Documentation/output/./latex

Fixes: e61a39ba ("[media] index.rst: Fix LaTeX error in interactive mode on Sphinx 1.4.x")
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>