panfrost: Derive UBO count from shader_info

Rather than checking against the bound constant buffers. Like
with num_textures, this eliminates a dependency of the shader descriptor
on the context, which matters especially for vertex/compute shaders.

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Reviewed-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6440>