panfrost,panvk: Make fixed_sysval_ubo < 0 mean compiler-assigned

panfrost,panvk: Make fixed_sysval_ubo < 0 mean compiler-assigned

In 3559efb9bf5c ("panfrost: Allow passing an explicit UBO index for the
sysval UBO"), an explicit UBO index was added and it was implicitly
assumed that it would be > num_ubos.  This was convenient because it
meant 0, the default for designated initializers, implicitly meant
compiler-assigned.  However, we're about to move the sysval UBO to 0
which breaks this assumption.   Also, we don't want the back-end
compiler to even look at num_ubos since it's meaningless in Vulkan.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/16276>