drm/msm/dpu: introduce the dpu_encoder_phys_* for writeback

Introduce the dpu_encoder_phys_* for the writeback interface
to handle writeback specific hardware programming.

changes in v4:
	- squash the encoder_phys_wb bits from [1]
	- since its a trivial change of a previously acked change
	  preserving the ack

[1] https://patchwork.freedesktop.org/patch/483099/?series=102964&rev=2



Signed-off-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/483532/
Link: https://lore.kernel.org/r/1650984096-9964-14-git-send-email-quic_abhinavk@quicinc.com


Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>