kgsl/drm: add mutexing with enable/event handler.

This patch adds mutex to avoid sync problem with user space.

Change-Id: I825bafbc0e91aa7029ff764a9d22b710f5382428
Signed-off-by: Eunchul Kim <chulspro.kim@samsung.com>
Signed-off-by: Mathew Karimpanal <kjmathew@codeaurora.org>
(cherry picked from commit 947f6327c61265b3845660c492755d8714f93599)