drm: get rid of direct property value access

For atomic drivers, we won't use the values array but instead shunt
things off to obj->atomic_get_property().  So to simplify things make
all read/write of properties values go through the accessors.

Signed-off-by: Rob Clark <robdclark@gmail.com>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>