msdkdec: Fix to not trigger SFC scaling when vpp at downstream

When there is vpp scaling downstream, we need to make sure SFC is not triggered because vpp may fall into passthrough mode which causes the decoder negotiation to create src caps with vpp scaled width/height.

This patch adds a first query using bitstream's original size with downstream to ensure SFC scaling starts to work only when downstream directly asks for a different size instead of through vpp.