panfrost: Add SSBO system value

panfrost: Add SSBO system value

For each SSBO index we get from Gallium/NIR, we need two pieces of
information in the shader:

1. The address of the SSBO in GPU memory. Within the shader, we'll be
accessing it with raw memory load/store, so we need the actual address,
not just an index.

2. The size of the SSBO. This is not strictly necessary, but at some
point, we may like to do bounds checking on SSBO accesses.

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>