drm/exynos/ipp: remove events during command cleaning

Events were removed only during stop command, as a result
there were memory leaks if program prematurely exited.
This patch fixes it.

Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Reviewed-by: Joonyoung Shim <jy0922.shim@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>