drm/msm/dpu: fix incorrect type for ret

drm/msm/dpu: fix incorrect type for ret

Change 'ret' from unsigned long to int, as storing negative error codes
in an unsigned long makes it never equal to -ETIMEDOUT, causing logical
errors.

Fixes: d7d0e73f7de3 ("drm/msm/dpu: introduce the dpu_encoder_phys_* for writeback")
Signed-off-by: Qianfeng Rong <rongqianfeng@vivo.com>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
Patchwork: https://patchwork.freedesktop.org/patch/671100/
Link: https://lore.kernel.org/r/20250826092047.224341-1-rongqianfeng@vivo.com
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>