drm/exynos/ipp: remove incorrect checks of list_first_entry result

list_first_entry does not return NULL on empty list so this check
does not make sense. Moreover there is already code which prevents calling
list_first_entry on empty lists.

Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>