drm/msm/dp: fix race condition during simulation

The simulation script can end abruptly during simulation
testing. This may result in buffers being released by the
script while the test may still be using them.
Use mutex lock to avoid multiple threads to allocate/deallocate
and access the buffers at the same time to avoid system instability.

CRs-Fixed: 2308439
Change-Id: Ia1c407d91afa98b4085d58cb45c8659206a0f990
Signed-off-by: Ajay Singh Parmar <aparmar@codeaurora.org>