drm/msm/dp: protect edid and dpcd buffers with mutex

The EDID and DPCD buffers are allocated and deallocated
in debug module while running in simulation mode. There
can be a race condition if multiple scripts are run and
result in double free of the buffers.

Protect the buffer allocation and deallocation functionalities
with a mutex so that such race conditions can be avoided.

Change-Id: Id00c0c95dc0151bf50389ec509ab8372c4ce5102
Signed-off-by: Ajay Singh Parmar <aparmar@codeaurora.org>
Signed-off-by: Tatenda Chipeperekwa <tatendac@codeaurora.org>