drm/msm/sde: disable write operation on eventlog dump node

sde event logger creates debugfs nodes to log events and
show them to end user. Disable write operation on dump node
because it is not required.

Change-Id: Ib9f7c82a6604384bca8b6ccfddbace527d195edb
Signed-off-by: Prashant Singh <prasin@codeaurora.org>