drm/exynos: remove unnecessary codes

This patch removes exynos_drm_crtc_cancel_page_flip call
when drm is closed because at that time, events will be released
by drm_events_release function.

Changelog v1:
- remove exynos_drm_crtc_cancel_page_flip function also because
  this funtion isn't used anymore.

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