drm: When reading connector mode tunables, list connectors

... and their associated tunables. This gives a way to know the list of
available connectors, no matter the driver.

The problem is that xrandr(1) can list connectors but it uses a
different naming.

MFC after:	1 week


git-svn-id: svn+ssh://svn.freebsd.org/base/head@274049 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f