input: synaptics_dsx: protect tmpbuf allocation

Protect tmpbuf from concurrent access by mutex.
BUG: 33555878
BUG: 33002026

Change-Id: Ia7eeb59ca7b626f416e2298b4b9ffd960fe909e4
Signed-off-by: Andrew Chant <achant@google.com>
Git-repo: https://android.googlesource.com/kernel/msm


Git-commit: e6430a4da1fb0212a546379eadbe986f629c3ae9
Signed-off-by: Venkata Prahlad Valluru <vvalluru@codeaurora.org>