drm/exynos: atomic phase 2: keep track of framebuffer pointer

Use drm_atomic_set_fb_for_plane() in the legacy page_flip path to keep
track of the framebuffer pointer and reference.

Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Reviewed-by: Joonyoung Shim <jy0922.shim@samsung.com>
Tested-by: Tobias Jakobi <tjakobi@math.uni-bielefeld.de>
Signed-off-by: Inki Dae <inki.dae@samsung.com>