drm/msm/dpu: check for null return of devm_kzalloc() in dpu_writeback_init()

Because of the possilble failure of devm_kzalloc(), dpu_wb_conn might
be NULL and will cause null pointer dereference later.

Therefore, it might be better to check it and directly return -ENOMEM.

Fixes: 77b001ac ("drm/msm/dpu: add the writeback connector layer")
Signed-off-by: Hui Tang <tanghui20@huawei.com>
Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Patchwork: https://patchwork.freedesktop.org/patch/512277/
Link: https://lore.kernel.org/r/20221119055518.179937-1-tanghui20@huawei.com


[DB: fixed typo in commit message]
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>