drm/imx: atomic phase 2 step 2: Track plane_state->fb correctly in ->page_flip

Use drm_atomic_set_fb_for_plane() in the legacy ->page_flip path to track
the pointer plane_state->fb correctly.

Signed-off-by: Liu Ying <gnuiyl@gmail.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>