decproxy2: add stream lock for decoder switching callback
decproxy2: add stream lock for decoder switching callback

Change-Id: I31a5a1e4780205bd5be6bd79581fa825cba54aa1
Reviewed-on: https://gpro.lgsvl.com/76473
Reviewed-by: Jeongseok Kim <0c03ab7a1b32f29697716389c3f9549845834271@lge.com>
Tested-by: Jeongseok Kim <0c03ab7a1b32f29697716389c3f9549845834271@lge.com>
Reviewed-by: Wonchul Lee <9841bdbec9dbb2acc2dcaaa1dab0491399f0e464@lge.com>

justinjoy/gst-plugins-cool,justinjoy/gst-plugins-cool