This is ported from the same code in the ati and intel drivers,

It uses the same option name as nvidia and the other DDXes to
disable tearing down outputs as it is hard to avoid racing with clients.

v2: address two issues with DeleteUnusedDP12 enabled, reported
by Daniel Martin,
a) check we have a mode_output before destroying it
b) only delete *unused* displays (thanks Aaron for clarifying)
so we check if the output has a crtc and if it does we don't
delete it.

v3: drop the option to delete unused displays, just encode
behaviour into the randr spec.

Signed-off-by: Dave Airlie <airlied@redhat.com>