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

In the Linux kernel, the following vulnerability has been resolved:

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.

Patchwork: https://patchwork.freedesktop.org/patch/512277/

[DB: fixed typo in commit message]