drm/exynos: mixer: fix chroma comment in vp_video_buffer()

The current comment sounds like the division op is done to
compensate for some hardware erratum. But the chroma plane
having half the height of the luma plane is just the way
NV12/NV21 is defined, so clarify this behaviour.

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