anv: only use VkSamplerCreateInfo::compareOp if enabled

anv: only use VkSamplerCreateInfo::compareOp if enabled

The spec says nothing about the validity of the compareOp field when
compareEnable is false.

v2: use vulkan enum to pick default value (Caio)

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Cc: <mesa-stable@lists.freedesktop.org>
Closes: https://gitlab.freedesktop.org/mesa/mesa/issues/2350
Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3387>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3387>