perf trace: Don't set the base timestamp using events without PERF_SAMPLE_TIME

perf trace: Don't set the base timestamp using events without PERF_SAMPLE_TIME

This was causing bogus values to be shown at the timestamp column:

Before:

  # trace --ev bpf-output/no-inherit,name=evt/ --ev /home/acme/bpf/test_bpf_trace.c/map:channel.event=evt/ usleep 10
  94631143.385 ( 0.001 ms): brk(                                     ) = 0x555555757000
  94631143.398 ( 0.003 ms): mmap(len: 4096, prot: READ|WRITE, flags: PRIVATE|ANONYMOUS, fd: -1) = 0x7ffff7ff6000
  94631143.406 ( 0.004 ms): access(filename: 0xf7df9e10, mode: R     ) = -1 ENOENT No such file or directory
  94631143.412 ( 0.004 ms): open(filename: 0xf7df8761, flags: CLOEXEC) = 3
  94631143.415 ( 0.002 ms): fstat(fd: 3, statbuf: 0x7fffffffd6b0     ) = 0
  94631143.419 ( 0.003 ms): mmap(len: 106798, prot: READ, flags: PRIVATE, fd: 3) = 0x7ffff7fdb000
  94631143.420 ( 0.001 ms): close(fd: 3                              ) = 0
  94631143.432 ( 0.004 ms): open(filename: 0xf7ff6640, flags: CLOEXEC) = 3
  <SNIP>

After:

  # trace --ev bpf-output/no-inherit,name=evt/ --ev /home/acme/bpf/test_bpf_trace.c/map:channel.event=evt/ usleep 10
  0.022 ( 0.001 ms): brk(                                     ) = 0x55d7668a6000
  0.037 ( 0.003 ms): mmap(len: 4096, prot: READ|WRITE, flags: PRIVATE|ANONYMOUS, fd: -1) = 0x7f8fbeb97000
  0.123 ( 0.083 ms): access(filename: 0xbe995e10, mode: R     ) = -1 ENOENT No such file or directory
  0.130 ( 0.004 ms): open(filename: 0xbe994761, flags: CLOEXEC) = 3
  0.133 ( 0.002 ms): fstat(fd: 3, statbuf: 0x7fff6487a890     ) = 0
  0.138 ( 0.003 ms): mmap(len: 106798, prot: READ, flags: PRIVATE, fd: 3) = 0x7f8fbeb7c000
  0.140 ( 0.001 ms): close(fd: 3                              ) = 0
  0.151 ( 0.004 ms): open(filename: 0xbeb97640, flags: CLOEXEC) = 3
  <SNIP>

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Milian Wolff <milian.wolff@kdab.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/n/tip-p7m8llv81iv55ekxexdp5n57@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>