drm: Unify handling of blob and object properties

They work exactly the same now, after the refcounting unification a bit
ago. The only reason they're distinct is backwards compat with existing
userspace.

Cc: Daniel Stone <daniels@collabora.com>
Reviewed-by: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20160829082757.17913-8-daniel.vetter@ffwll.ch