drm/exynos: fix vblank bug.

In case that vblank_disable_allowed is 1, the problem that manager->pipe
could be -1 at vsync interrupt handler could be induced so this patch fixes
that.

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