drm: Always enable atomic API

Now that the interface has been proven by a port of Weston (using all
atomic features including TEST_ONLY), remove the module parameter
guarding the atomic API from being exposed, and let it run free in the
wild.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>