nvdec: Fix possible frame drop on EOS

On eos, baseclass videoencoder call finish() vfunc instead of drain()
18 jobs for fix-nvdec-framedrop in 76 minutes and 2 seconds (queued for 5 minutes and 14 seconds)
Status Job ID Name Coverage
  Preparation
passed #409243
gst indent

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passed #409242
manifest

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  Build
passed #409249
build cerbero android universal

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passed #409250
build cerbero cross win32

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build cerbero cross win64

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passed #409248
build cerbero fedora x86_64

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manual #409253
gst-ios-12.2 allowed to fail manual
build cerbero ios universal
passed #409252
gst-macos-10.14
build cerbero macos x86_64

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build fedora x86_64

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passed #409245
build nodebug fedora x86_64

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passed #409246
build static fedora x86_64

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passed #409247
build static nodebug fedora x86_64

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  Test
passed #409498
check fedora

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passed #409255
integration testsuites fedora

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passed #409256
valgrind bad

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failed #409254
check fedora

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  Integrate
passed #409257
android universal examples

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passed #409258
documentation

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