drm: Allow drivers to modify plane_state in prepare_fb/cleanup_fb

The drivers have to modify the atomic plane state during the prepare_fb
callback so they track allocations, reservations and dependencies for
this atomic operation involving this fb. In particular, how else do we
set the plane->fence from the framebuffer!

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20160818180017.20508-1-chris@chris-wilson.co.uk