drm/exynos: use KMS version of DRM vblanks functions

Get rid of legacy DRM vblank function that are less clear to use.
The new ones basically requires only the crtc as parameters.

It also clean ups exynos_drm_crtc_finish_pageflip() parameters as a
consequence.

Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Reviewed-by: Joonyoung Shim <jy0922.shim@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>