The current code was ignoring the end of update for all overlay planes,
caring only for the primary plane update in case of pageflip.

This change adds a change to start to check for pending updates for all
planes through exynos_plane->pending_fb. At the start of plane update the
pending_fb is set with the fb to be shown on the screen. Then only when to
fb is already presented in the screen we set pending_fb to NULL to
signal that the update was finished.
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Signed-off-by: Inki Dae <inki.dae@samsung.com>

fixup! drm/exynos: check for pending fb before finish update