msm: mdss: fix race condition during mdp debugfs release

Fix race condition in the release of the mdp debugfs functions
panel_debug_base_release and mdss_debug_base_release by adding
the lock for unpreempted freeing of the buffer so that multiple
concurrent processes cannot affect the release which can possibly
lead to use-after-free operation on the buffer.

Change-Id: I9586081b65ae2eb0e7f6e30c606ee748ae9ef7e8
Signed-off-by: Harsh Sahu <hsahu@codeaurora.org>