anv: VkDescriptorSetLayoutBinding can have descriptorCount == 0

From Vulkan spec:

"descriptorCount is the number of descriptors contained in the binding,
accessed in a shader as an array. If descriptorCount is zero this
binding entry is reserved and the resource must not be accessed from
any stage via this binding within any pipeline using the set layout."

Fixes:

dEQP-VK.binding_model.descriptor_update.empty_descriptor.uniform_buffer

Signed-off-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Cc: mesa-stable@lists.freedesktop.org
(cherry picked from commit e63adf8b)