drm/exynos: fix lockdep for event_lock wrt. vbl_time_lock

Currently the exynos driver calls drm_vblank_off() with the event_lock
held, while drm_vblank_off() will lock vbl_time and vblank_time_lock.
This lock dependency chain conflicts with the one in drm_handle_vblank()
where we first lock vblank_time_lock and then the event_lock.

Fix this by removing the above drm_vblank_off() calls which are in fact
never executed: drm_dev->vblank_disable_allowed is only ever non-zero
during driver init, until it's set in {fimd,vidi}_subdrv_probe. Both the
driver init and open code is protected by drm_global_mutex, so the
earliest page flip ioctl can happen only after vblank_disable_allowed is
set to 1. Thus {fimd,vidi}_finish_pageflip - with pending flip events -
will always get called with vblank_disable_allowed being 1.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>