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
16 jobs for master in 50 minutes and 14 seconds (queued for 13 seconds)
Status Job ID Name Coverage
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gst indent

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manifest

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build cerbero android universal

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gst-ios-12.2 allowed to fail manual
build cerbero ios universal
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android universal examples

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documentation

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