freedreno/log: avoid duplicate ts's

freedreno/log: avoid duplicate ts's

In cases where `fd_log()`/`fd_log_stream()` are called multiple times
back-to-back, just use the timestamp of the first trace.

This seems to avoid some occasional GPU hangs I was seeing with logging
enabled.  Although not exactly sure the reason for the hangs.  (Looks
like GPU hangs *after* all the cmdstream is processed, according to
crashdec.)

Signed-off-by: Rob Clark <robdclark@chromium.org>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4366>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4366>