disp: msm: sde: allow kthread init for off work during cwb

During transition to cwb, kthread initialisation has to
be done for the corresponding encoder off work so that
the correct worker thread is used for this particular
work. There can be scenarios where a cwb commit is
received after a writeback session and the worker
associated with the off work is still assigned to
old crtc's worker resulting in a mismatch when
this work is queued. Add support to handle such
scenarios.

Change-Id: I6080025e799977827f4d0f4ab7eb93c6644f981e
Signed-off-by: Krishna Manikandan <mkrishn@codeaurora.org>