anv: fix descriptor limits on gen8

anv: fix descriptor limits on gen8

Later generations support bindless for samplers, images, and buffers and
thus per-stage descriptors are not limited by the binding table size.
However, gen8 doesn't support bindless images and thus needs to report a
lower per-stage limit so that all combinations of descriptors that fit
within the advertised limits are reported as supported by
vkGetDescriptorSetLayoutSupport.

Fixes test dEQP-VK.api.maintenance3_check.descriptor_set
Fixes: 79fb0d27f3 ("anv: Implement SSBOs bindings with GPU addresses in the descriptor BO")
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>