drm/msm/dpu: Assign missing writeback log_mask

drm/msm/dpu: Assign missing writeback log_mask

The WB debug log mask ended up never being assigned, leading to writes
to this block to never be logged even if the mask is enabled in
dpu_hw_util_log_mask via debugfs.

Fixes: 84a33d0fd921 ("drm/msm/dpu: add dpu_hw_wb abstraction for writeback blocks")
Signed-off-by: Marijn Suijten <marijn.suijten@somainline.org>
Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/533860/
Link: https://lore.kernel.org/r/20230418-dpu-drop-useless-for-lookup-v3-1-e8d869eea455@somainline.org
Signed-off-by: Abhinav Kumar <quic_abhinavk@quicinc.com>