drm: add atomic properties

Once a driver is using atomic helpers for modeset, the next step is to
switch over to atomic properties.  To do this, make sure that any
modeset objects have their ->atomic_{get,set}_property() vfuncs suitably
populated if they have custom properties (you did already remember to
plug in atomic-helper func for the legacy ->set_property() vfuncs,
right?), and then set DRIVER_ATOMIC bit in driver_features flag.

A new cap is introduced, DRM_CLIENT_CAP_ATOMIC, for the purposes of
shielding legacy userspace from atomic properties.  Mostly for the
benefit of legacy DDX drivers that do silly things like getting/setting
each property at startup (since some of the new atomic properties will
be able to trigger modeset).

Signed-off-by: Rob Clark <robdclark@gmail.com>
[danvet: Squash in fixup patch to check for DRM_MODE_PROP_ATOMIC
instaed of the CAP define when filtering properties. Reported by
Tvrtko Uruslin, acked by Rob.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>