plugins/screencast: Avoid resetting m_dmabufParams when resizing the stream
ScreenCastStream::onStreamAddBuffer() can be called before the stream params are updated. This is problematic because that function expects a valid m_dmabufParams.
The reason why m_dmabufParams is reset is to force the onStreamParamChanged() function to check whether dmabufs can be still allocated with the new size.
(cherry picked from commit e2611d71)