Delete Mesa Classic

As proposed, here is the concrete removal of the Mesa classic drivers and infrastructure. I've tested with Meson and things continue to compile tests pass. I have tried to break the commits up into logical chunks, removing one driver at a time, then common code, then libmesa classic itself.

There are the following additional changes

• move the dri megadriver code to gallium/auxilliary. I'm not sure where else to put it, it's used to two gallium targets
• remove the core mesa dispatch sanity test. This would require a pretty substantial rework to still be useful, as the code that it uses for setup (but not the code it tests) is being deleted. I suspect that the code tested will be refactored out in the near future, so that work wouldn't be that useful.
• remove classic xlib. I know that the gallium-xlib code isn't quite as good as classic xlib, but it's not a super common feature anyway.]

I have moved the PCI IDs that both iris and i965 supports into the iris pci_ids header, and removed them from i965. I have left the actual header in place, along with the rest of the Intel GFX version 4-7.5 code, as I assume that the crocus effort can use that.

I've done my best to keep the Android.mk working, but I'm sure I've missed something.

Finally, if there are any requests to re-author any patches, please let me know and I'll re-author them.

This should not land before the 21.1 branch point, but shortly after instead.