drm/exynos: Use drm_fb_helper_lastclose() and _poll_changed()

drm/exynos: Use drm_fb_helper_lastclose() and _poll_changed()

This driver can use drm_fb_helper_lastclose() as its .lastclose callback.
It can also use drm_fb_helper_output_poll_changed() as its
.output_poll_changed callback.

Cc: Inki Dae <inki.dae@samsung.com>
Cc: Joonyoung Shim <jy0922.shim@samsung.com>
Cc: Seung-Woo Kim <sw0312.kim@samsung.com>
Cc: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Inki Dae <inki.dae@samsung.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171205182504.41923-5-noralf@tronnes.org