OMAPDSS: DISPC: Allow both upscaling and downscaling of chroma

In the function dispc_plane_set_scaling_uv(), create a parameter which tells if
we want to upscale or downscale the chroma plane.

Downscaling of chroma is required by writeback pipeline for converting the input
YUV444 color format to YUV422 or NV12.

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