drm/exynos: Remove event cancelling from postclose

drm/exynos: Remove event cancelling from postclose

The core takes care of this now. And since kfree(NULL) is ok we can
simplify the function even further now.

Note: There's another spin on this patch, but for different reasons,
in-flight already: http://www.spinics.net/lists/dri-devel/msg97922.html

Cc: Inki Dae <inki.dae@samsung.com>
Acked-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Acked-by: Inki Dae <inki.dae@samsung.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1453756616-28942-6-git-send-email-daniel.vetter@ffwll.ch