drm/msm/rd: fix crash with long process cmdlines

The [v]snprintf() functions return the size that *would have* been
written into the buffer, rather than the size *actually* written.
Which results in us trying to memcpy() past the end of the stack.

What we really want is [v]scnprintf().

Signed-off-by: Rob Clark <robdclark@gmail.com>