drm/exynos: modify wait_for_vblank of mixer

drm/exynos: modify wait_for_vblank of mixer

It is more optimium to use wait queues while waiting for vsync so
that the current task is put to sleep. This way, the task wont
hog the CPU while waiting. We use wait_event_timeout and not
an interruptible function since we dont want the function to exit
when a signal is pending (e.g. drm release). This patch modifies
the wait for vblank function of mixer.

Signed-off-by: Prathyush K <prathyush.k@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>