drm/exynos: remove wait queue for pending page flip

Exynos atomic commit procedures already does this job of waiting for
pending updates to finish, that means using pending_flip_queue is
pointless now because the disable CRTC procedure will never happen
during a page_flip.

Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Signed-off-by: Inki Dae <inki.dae@samsung.com>