amdgpu, radeonsi fixes for the EXT_external_objects extension to work again

While working on MR !4337 (merged) I've noticed that the AMD implementation is not working anymore (Piglit tests under tests/spec/ext_external_objects/ directory don't pass, although they pass on Intel and Nvidia). Moreover, importing optimal tiling Vulkan textures was not working (I think that was intentionally left for the future) and importing external buffers was causing errors.

This branch is an attempt to fix these issues. I've tested it with Piglit master (tests/spec/ext_external_objects).

There are some things that still need fixing (but this applies to all mesa drivers I've tested so far) like importing an external depth buffer and importing an external stencil buffer. There are 2 tests for that (that don't pass) in piglit!329 (merged) and piglit!330 (closed).