drm/msm/sde: ensure proper cleanup of WB pipeline during disable

Ensure to follow proper cleanup sequence of WB interface even when
some hw blocks are not available for low end targets.

Change-Id: Ia8c0eb033b06d0e522f94971adce7aefaf6b2a7f
Signed-off-by: Kalyan Thota <kalyant@codeaurora.org>
Signed-off-by: Abhijith Desai <desaia@codeaurora.org>