drm/crtc: Use drm_mode_object_put() in __drm_framebuffer_unregister()

The function __drm_framebuffer_unregister() has boilerplate code to drop idr
reference.  Let's replace it with drm_mode_object_put() to simplify the code.

Signed-off-by: Liu Ying <gnuiyl@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1456716070-2602-1-git-send-email-gnuiyl@gmail.com