virgl: Don't allow creating staging pipe_resources

Staging buffers are now created directly by the virgl_staging_mgr. We
don't need to support creating staging pipe_resources.

Signed-off-by: Alexandros Frantzis <alexandros.frantzis@collabora.com>
Reviewed-by: Chia-I Wu <olvaffe@gmail.com>
10 jobs for !1178 with virgl-staging-mgr in 8 minutes and 29 seconds (queued for 1 second)
latest detached
Status Job ID Name Coverage
  Containers Build
passed debian #386488

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  Build+Test
passed meson-clang #386490

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passed meson-clover #386494

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passed meson-main #386493

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passed meson-swr-glvnd #386489

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passed meson-vulkan #386495

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passed scons-llvm #386497

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passed scons-nollvm #386496

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passed scons-swr #386491

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passed scons-win64 #386492

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