drm/exynos/vidi: use timer for vblanks instead of sleeping worker

VIDI driver uses fake vblank handler to generate vblank events.
It was implemented using worker which slept for vblank time, additionally
it did not work if there were no page flips. The patch replaces it with
timer, uses drm_crtc_vblank_(on|off) helpers to manage it and fixes
behavior for non-page-flip cases.
This change allows further improvements of vblank in exynos-drm framework.

Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Reviewed-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Signed-off-by: Inki Dae <inki.dae@samsung.com>