mm-camera2:isp2: Release buffer lock after use

At start axi stream, we are acquiring the
buffer lock and releasing after completing
the stream configuration operations. In case
of live snapshot,this is causing the buffer
operations to halt and leading to sof freeze.

Change-Id: I7f57c8befc85c105a5690ac71e0276f42b9f1929
Signed-off-by: Meera Gande <mgande@codeaurora.org>