drm/exynos: soft reset mixer before reconfigure after power-on

Mixer soft reset is a recommended step before reconfiguring
the mixer after power on. Mixer looses the previous state of
DMAs if soft reset. This is the recommendation from the
hardware team.

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