drm: samsung: add lock when dumping dpu event logs

There is race condition between read and write dpu event logs. Add lock
to protect dpu event log content to avoid race issue.

Bug: 209878019
Test: dump dpu event logs continuously and do some operators at same
      time, including on/off, video, camera, chrome, youtube
Test: simulate underrun to dump dpu event logs automatically
Change-Id: Idcd77c6f2c230557be4f0851d4bcd55bb6d115df
Signed-off-by: Midas Chien <midaschieh@google.com>