gstreamer/pad: Factor out PadProbeInfo handling from pad probe trampoline into separate functions
This makes that part of the code non-generic and thus allows the compiler to not put a copy of it into every caller with a different closure.
For a test with 3 pad probes this overall reduced the number of LLVM IR lines needed for the pad probes to about 8.5% of what it was before (4485 -> 381 lines).