drm/exynos: remove unnecessary type conversion of hdmi and mixer

When the void pointer type variable is assigned to the specific pointer
type variable, don't need to do type conversion.

Signed-off-by: Joonyoung Shim <jy0922.shim@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>