drm/exynos: mixer: avoid Oops in vp_video_buffer()

If an interlaced video mode is selected, a IOMMU pagefault is
triggered by vp_video_buffer().

Fix the most apparent bugs:
- pitch value for chroma plane
- divide by two of height and vpos of source and destination

Signed-off-by: Tobias Jakobi <tjakobi@math.uni-bielefeld.de>
[ a.hajda: Halved also destination height and vpos, updated commit message ]
Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>