freedreno: Trim num_ubos to just the ones we haven't lowered to constbuf.

With the upcoming LDC usage in the GL driver, we don't want to be
uploading descriptors for every UBO when they aren't actually in use.
Trimming NIR's num_ubos will avoid that, and cleans up num_ubo handling
elsewhere right now.

Part-of: <mesa/mesa!4858>