drm/msm/sde: add mutex to the sde resource manager

Fix serialization of access to the SDE resource manager by
adding mutex protection to its external APIs.

Change-Id: I469a1c7b37d4a2f115443bdc308d0236d786fc68
Signed-off-by: Lloyd Atkinson <latkinso@codeaurora.org>
Signed-off-by: Lihui Wen <lwen@codeaurora.org>