drm/mode: move framebuffer reference into object.

This is the initial code to add references to some mode objects.
In the future we need to start reference counting connectors so
firstly I want to reorganise the code so the framebuffer ref counting
uses the same paths.

This patch shouldn't change any functionality, just moves the kref.

[airlied: move kerneldoc as well]
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>