drm/msm/gpu: Fix potential double-free

If userspace was calling the MSM_SET_PARAM ioctl on multiple threads to
set the COMM or CMDLINE param, it could trigger a race causing the
previous value to be kfree'd multiple times.  Fix this by serializing on
the gpu lock.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Fixes: d4726d77 ("drm/msm: Add a way to override processes comm/cmdline")
Patchwork: https://patchwork.freedesktop.org/patch/517778/
Link: https://lore.kernel.org/r/20230110212903.1925878-1-robdclark@gmail.com