OMAPDSS: DISPC: Downscale chroma if plane is writeback

When converting YUYV444 content to YUV422 or NV12 formats through writeback
pipeline, the scaler needs to downscale the chroma plane. Ensure that chroma
is downscaled when the pipeline is writeback.

Signed-off-by: Archit Taneja <archit@ti.com>