drm/msm/dpu: use drmm_writeback_connector_init()

drm/msm/dpu: use drmm_writeback_connector_init()

Use drmm_writeback_connector_init() in order to initialize writeback
connector instance.

Reviewed-by: Louis Chauvet <louis.chauvet@bootlin.com>
Reviewed-by: Suraj Kandpal <suraj.kandpal@intel.com>
Reviewed-by: Jessica Zhang <jessica.zhang@oss.qualcomm.com>
Patchwork: https://patchwork.freedesktop.org/patch/669506/
Link: https://lore.kernel.org/r/20250819-wb-drop-encoder-v3-4-b48a6af7903b@oss.qualcomm.com
[DB: fixed commit message]
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>