panfrost: Allow passing an explicit UBO index for the sysval UBO

panfrost: Allow passing an explicit UBO index for the sysval UBO

UBO index assignment is a bit special in Vulkan, it's based on the
descriptor set layout, which doesn't know about shaders' internal UBOs
(our sysval UBOs). Extend the backend compilers so we can place sysval
UBOs where we want: after all explicit UBOs.

Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Alyssa Rosenzweig <alyssa@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/9517>