msdk: don't share context between msdkvpp and msdkenc
msdkenc supports CSC implicitly, so it is possible that two VPP processes are required when a pipeline contains msdkvpp and msdkenc. Before this fix, msdkvpp and msdkenc may share the same context, hence the same mfx session, which results in MFX_ERR_UNDEFINED_BEHAVIOR in MSDK because a mfx session has at most one VPP process only
This fixes the broken pipelines below:
gst-launch-1.0 videotestsrc ! video/x-raw,format=I420 ! msdkh264enc !
msdkh264dec ! msdkvpp ! video/x-raw,format=YUY2 ! fakesink
gst-launch-1.0 videotestsrc ! msdkvpp ! video/x-raw,format=YUY2 !
msdkh264enc ! fakesink