drm/msm: protect register dumps and debugfs with mutex

Add a mutex to serialize sde debug register dumps for
back-to-back dumps, as well as any debugfs access.

Change-Id: I7f6186575df021ce48e18ea980f9c2108b756283
Signed-off-by: Lloyd Atkinson <latkinso@codeaurora.org>
Signed-off-by: Lakshmi Narayana Kalavala <lkalaval@codeaurora.org>