drm/exynos: enable vsync interrupt while waiting for vblank

mixer_wait_for_vblank function expects that the upcoming
vsync interrupt handler routine will clear the
wait_vsync_event atomic variable.

For this to happen, interrupts should be enabled and
disabled properly.

Signed-off-by: Rahul Sharma <rahul.sharma@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>