media: ngene: Replace semaphore stream_mutex with mutex

The semaphore 'stream_mutex' is used as a simple mutex, so
it should be written as one. Also moving the mutex_[lock/unlock]
to the caller as it is anyway locked at the beginning of the
callee thus avoiding repetition.

Signed-off-by: Binoy Jayan <binoy.jayan@linaro.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>