modesetting: Fix broken manpage in autoconf build

The autoconf build for the modesetting driver still relied on
xorg-macros.m4 for string replacements and did not include the
top-level manpages.am.  As a result, no substitutions took place after
commit 2e497bf8.

This should be a candidate for the 1.20 branch.

Reviewed-by: Michel Dänzer <mdaenzer@redhat.com>
(cherry picked from commit de0d39f8)