drm: exynos: removed warning due to missing typecast for mixer driver data

Removing the warning by adding proper type casting where local pointer
variable of type mixer driver data is assigned with void pointer.

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