drm/msm/dpu: add writeback blocks to DPU RM

drm/msm/dpu: add writeback blocks to DPU RM

Add writeback blocks to DPU resource manager so that
the encoders can directly request them through RM.

changes in v4:
	- absorb dpu_rm.h header change from [1]
	- since its a trivial change absorbed from an approved
	  patch, preserving the previous ack on this

[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/483508/
Link: https://lore.kernel.org/r/1650984096-9964-8-git-send-email-quic_abhinavk@quicinc.com
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>