anv/formats: Advertise linear sampling on depth formats

They've all supported it since either forever or Iron Lake which is
equivalent to forever for Vulkan.

From Kenneth Graunke's GitLab review:

    "Linear blending of depth buffer data is usually fairly nonsense
    (something's 2 meters away?  another thing's 6 meters away?  let's
    just report 4 meters?)...but it's definitely a thing we can do, so
    we may as well let apps do it, and trust them not when it doesn't
    make sense."

Cc: mesa-stable@lists.freedesktop.org
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Part-of: <!9110>
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