drm/exynos: stop mixer before gating clocks during poweroff

Mixer should be power gated only after it is gracefully stopped.
The recommended sequence is to Stop the mixer and wait till
it enters to IDLE state before gating the clocks and power to
the mixer.

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