drm/msm/sde: cleanup writeback phys_enc structures during wb disable

Hardware structures for writeback ctl and cdm are set
to null during wb disable, to prevent crtc mode change
on primary during subsequent wfd and cwb sessions.

Change-Id: I7536203761c615c37c8633d1621951475895400a
Signed-off-by: Prashant Singh <prasin@codeaurora.org>