OMAPDSS: DISPC: Don't set chroma resampling bit for writeback

The bit YUVCHROMARESAMPLING isn't there for writeback in DISPC_WB_ATTRIBUTES2.
It isn't there because we don't upsample chroma like for video pipelines, we
downsample chroma in writeback to get YUV422 or NV12 formats from the YUV444
input.

Ignore this bit in dispc_ovl_set_scaling_uv() if the plane is OMAP_DSS_WB.

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