man: add drm.7 overview page

The drm.xml file compiles to drm.7 and is meant as a global overview page
for libdrm. It is targeted to new users of libdrm and redirects to all
other main man-pages.

Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>