drm: Remove drm_mode_object->atomic_count

It's only used in drm_mode_object_get_properties, and we can compute
it there directly with a bit of code shuffling.

Reviewed-by: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20160829082757.17913-4-daniel.vetter@ffwll.ch