panvk: Put the sysval and push const UBOs at fixed indices

panvk: Put the sysval and push const UBOs at fixed indices

In theory, this may cost us a tiny bit of descriptor space but in
practice, given that the viewport transform is a sysval, we'll always
need it for 3D and given that SSBO pointers live there, we'll basically
always need it for compute.  It also makes a lot of things simpler.
We're about to start using the sysval UBO directly in our descriptor set
code and knowing the index up-front is really nice.

Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/16276>