drm/msm: Fix potential invalid ptr free

The error path cleanup expects that chain and syncobj are either NULL or
valid pointers.  But post_deps was not allocated with __GFP_ZERO.

Fixes: ab723b7a ("drm/msm: Add syncobj support.")
Signed-off-by: Rob Clark <robdclark@chromium.org>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Patchwork: https://patchwork.freedesktop.org/patch/523051/
Link: https://lore.kernel.org/r/20230215235048.1166484-1-robdclark@gmail.com