drm/msm/dpu: Fix uninitialized variable in dpu_crtc_kickoff_clone_mode()

drm/msm/dpu: Fix uninitialized variable in dpu_crtc_kickoff_clone_mode()

After the loop there is a check for whether "wb_encoder" has been set
to non-NULL, however it was never set to NULL.  Initialize it to NULL.

Fixes: ad06972d5365 ("drm/msm/dpu: Reorder encoder kickoff for CWB")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/641631/
Link: https://lore.kernel.org/r/f8ba03dc-0f90-4781-8d54-c16b3251ecb1@stanley.mountain
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>