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.
3 jobs for modesetting-manpage in 9 minutes and 13 seconds (queued for 20 seconds)
Status Job ID Name Coverage
  Docker Image
passed #689040
debian-testing

00:00:36

 
  Build And Test
passed #689041
autotools-build-and-test

00:08:11

passed #689042
meson-build-and-test

00:01:49