drm/komeda: Fix off-by-1 when with readback conn due to rounding

When setting up a redback conenctor that writes data back to memory
rather than to an actual output device (HDMI etc.), rounding was ses
to round-down. As the DPU uses a higher internal number of bits when
generating a color value, this round-down back to 8bit ended up with
everything being off-by one. e.g. #ffffff became #fefefe. This sets
rounding to "round" so things end up correct by turning on the round
flag (LW_TRC).

Signed-off-by: Carsten Haitzler <carsten.haitzler@arm.com>