We want to get rid of plane->fb on atomic drivers. Stop looking at it.

Daniel pointed out that the drm_framebuffer_put() in the plane cleanup
indicates that the driver doesn't shut things down cleanly. To do
that we should be able to just call drm_atomic_helper_shutdown(). Not
really sure the current cleanup sequence is actually sane, but whatever.

v2: Replace the drm_framebuffer_put() with
    drm_atomic_helper_shutdown() (Daniel)

Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180405151400.11326-10-ville.syrjala@linux.intel.comReviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>