OMAPDSS: DISPC: Configure color conversion coefficients for writeback

Writeback pipeline receives RGB data from one of the overlays or one of the
overlay managers. If the target color mode is YUV422 or NV12, we need to convert
the RGB pixels to YUV. The scaler in WB then converts it to the target color
mode.

Hence, the color conversion coefficients that need to be programmed are the ones
which convert a RGB24 pixel to YUV444. Program these coefficients for writeback
pipeline.

Rearrange the code a bit to configure different coefficients for overlays and
writeback.

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