drm/msm/sde: limit debugfs permission for register dumping node

Remove write support in debugfs node used to dump register values.
Only read permission is required for this node.

Change-Id: I6d5ceb1d2b51a974403684a067995c323b69c7bb
Signed-off-by: Lakshmi Narayana Kalavala <lkalaval@codeaurora.org>