msm: wfd: Protect vb2 call with locks

The vb2 framework is only somewhat threadsafe.  Hence we need to
functions being called from multithreaded and preemptable contexts with
a mutex.

Change-Id: Ia31f937438540c38b2c65dd3b568981e1c526743
Signed-off-by: Deva Ramasubramanian <dramasub@codeaurora.org>