drm/msm/dpu: enable writeback on SM6125

Enable WB2 hardware block, enabling writeback support on this platform.

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Patchwork: https://patchwork.freedesktop.org/patch/570193/
Link: https://lore.kernel.org/r/20231203003203.1293087-4-dmitry.baryshkov@linaro.org