drm/exynos: fixed page flip issue.

with vblank_refcount = 1, there was the case that drm_vblank_put
is called by specific page flip function so this patch fixes the
issue.

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