drm/msm/dpu: don't mix devm and drmm functions

In the Linux kernel, the following vulnerability has been resolved: drm/msm/dpu: don't mix devm and drmm functions Mixing devm and drmm functions will result in a use-after-free on msm driver teardown if userspace keeps a reference on the drm device: The WB connector data will be destroyed because of the use of devm_kzalloc()), while the usersoace still can try interacting with the WB connector (which uses drmm_ functions). Change dpu_writeback_init() to use drmm_. Patchwork: https://patchwork.freedesktop.org/patch/722656/