drm/msm/dpu: add dpu_hw_wb abstraction for writeback blocks

Add the dpu_hw_wb abstraction to program registers related to the
writeback block. These will be invoked once all the configuration
is set and ready to be programmed to the registers.

changes in v3:
	- start using the common struct dpu_hw_cdp_cfg
	- leave a comment about DPU non-DPU_WB_QOS_8LVL chipsets

Signed-off-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/483505/
Link: https://lore.kernel.org/r/1650984096-9964-7-git-send-email-quic_abhinavk@quicinc.com
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>