drm/exynos: Remove custom FB helper deferred setup

The FB helper core now supports deferred setup, so the driver's custom
implementation can be removed.

v2: Drop NULL check, drm_fb_helper_hotplug_event handles that already.

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: Thierry Reding <treding@nvidia.com> (v1)
Reviewed-by: Inki Dae <inki.dae@samsung.com>
Tested-by: Inki Dae <inki.dae@samsung.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20170706130023.28417-4-daniel.vetter@ffwll.ch
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>