drm: Don't prepare or cleanup unchanging frame buffers [v3]

When reconfiguring a plane position (as in moving the cursor), the
frame buffer for the cursor isn't changing, so don't call the prepare
or cleanup driver functions.

This avoids making cursor position updates block on all pending rendering.

v3: use drm_atomic_helper_framebuffer_changed in both prepare and
    cleanup phases instead of keeping state in the plane.

cc: dri-devel@lists.freedesktop.org
cc: David Airlie <airlied@linux.ie>
cc: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Keith Packard <keithp@keithp.com>
[danvet: Rebase onto 4.8]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>