drm: Introduce drm_framebuffer_assign()

In a couple of places currently, and with the intent to add more, we
update a pointer to a framebuffer to hold a new fb reference (evicting
the old).

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20161125153231.13255-2-chris@chris-wilson.co.uk