drm/exynos: init vblank with real number of crtcs

Initialization of vblank with MAX_CRTC caused attempts
to disabling vblanks for non-existing crtcs in case
drm used fewer crtcs. The patch fixes it.

Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>