anv: Support B8G8R8A8_UNORM storage images

Even though B8G8R8A8_UNORM isn't one of the classic storage image
formats, it is the usual swapchain image format on Linux so it's helpful
to support it for storage.  Because SPIR-V provides no enums for BGRA
formats, it can only be used without a format which, on ANV, means it
must be a write-only image.  However, since the intended use-case is for
writing to a swapchain image from a compute shader, this doesn't seem
onerous.

v2: Only expose support based of driconf setting (Lionel)

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Status Job ID Name Coverage
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manual #4909364
aarch64 manual
arm_build
manual #4909365
manual
arm_test-base
manual #4909361
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aarch64
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x86_build
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meson-clang
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meson-release
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meson-vulkan
 
  Scons
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allowed to fail
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aarch64
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kvm
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llvmpipe-traces
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mesa-ci-aarch64-lava-collabora
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mesa-ci-aarch64-lava-collabora
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mesa-ci-aarch64-lava-collabora
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mesa-ci-aarch64-lava-collabora
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