drm/exynos: check for pending fb before finish update

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