drm/msm/dpu: make changes to dpu_encoder to support virtual encoder

drm/msm/dpu: make changes to dpu_encoder to support virtual encoder

Make changes to dpu_encoder to support virtual encoder needed
to support writeback for dpu.

changes in v4:
	- squash dpu_encoder pieces from [1]

changes in v5:
	- none

changes in v6:
	- fix the comment about intf_idx and wb_idx
	- add the condition for valid phys_enc with intf_idx
	  and wb_idx

[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/483514/
Link: https://lore.kernel.org/r/1650984096-9964-11-git-send-email-quic_abhinavk@quicinc.com
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>