drm/exynos: mixer: also allow NV21 for the video processor

All the necessary code is already there, just need to
handle the format in the switch statement.

Signed-off-by: Tobias Jakobi <tjakobi@math.uni-bielefeld.de>
Acked-by: Joonyoung Shim <jy0922.shim@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>