msm: sde: fix deadlock issue in concurrency test

When enabling early display, disabling weston service and launching
DRM_test, system looks to hang when DRM_test wants to open drm node.
The hang is caused by mutex deadlock.
So refine the code and remove the mutex lock to fix the problem.

Change-Id: I91d0c4e4418afde38c98a6294e49491403cdfe74
Signed-off-by: Guchun Chen <guchunc@codeaurora.org>
Signed-off-by: Yujun Zhang <yujunzhang@codeaurora.org>