drm/exynos: support drm_wait_vblank feature for VIDI

this patch adds drm_wait_vblank support to Virtual Display module
so user can use DRM_IOCT_WAIT_VBLANK ioctl with this patch.
for this, you should set _DRM_VBLANK_EXYNOS_VIDI flags to
vblwait->request.type

Signed-off-by: Inki Dae <inki.dae@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>