drm/msm: Add missing put_task_struct() in debugfs path

Fixes: 25faf2f2

 ("drm/msm: Show process names in gem_describe")
Signed-off-by: Rob Clark <robdclark@chromium.org>
Link: https://lore.kernel.org/r/20220317184550.227991-1-robdclark@gmail.com