r300: drop VDPAU support

This MR is intended to facilitate discussion what to do with the r300 VDPAU support. I'm thus CCing r300 users to seek some feedback (tried to CC anyone who reported a r300 bug or committed a r300 patch in the last year or so, sorry for spam to anyone not interested).

As far as I can say, it is currently broken, all my tries to use it with the vlc ended in crash or assertion fault. Dunno how to use it with mplayer, so the only thing I can get working is vdpauinfo. It might also get further broken when we remove the TGSI-only codepaths and start depending on some ntt behavior which I plan to do soon, so that is why I want to bring this up now. Additionally because it is mpeg2 only, all potentially affected machines can likely just use CPU and its unclear if there are any potential power-saving benefits of using shader-based mpeg2 vs the CPU anyway.

There are also no recent bug reports which (considering its likely broken) I would read as nobody cares. Potential bisect to check when this worked for the last time would be valuable, but its pita to build old mesa on new system and I was never interested enough to do so. All I know is it worked around 2011 according to old Phoronix articles.

However, I'm not really experienced testing/using VDPAU so I can't rule out there are some scenarios (other software, etc.) where it works. So if you care about r300 VDPAU and you have some usecase where it is useful for you (or worked in some past so that it can be actually bisected and potentially fixed) please speak up. Otherwise it will get removed soon.

CC @gawin @anholt @mareko @okias @lorn10 @ernsteiswuerfel @kas1e @blink @rsalvaterra @timemaster5 @dreamlayers @jwrdegoede @bitstreamout @illwieckz @MegWATTT @claudeha @Nyaaori @ultra0