drm/vram-helpers: Set plane fence for display update

Calling the VRAM helper's prepare_fb() helper now sets the plane's
fence object. This will be useful for PRIME support. VRAM helpers
don't support buffer sharing ATM, so for now there are no drivers
requiring this change.

v2:
	* removed a TODO comment about buffer synchronization

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20200331092740.29282-1-tzimmermann@suse.de