freedreno/ir3: drop shader->num_ubos

freedreno/ir3: drop shader->num_ubos

The only difference between this and `const_state->num_ubos` was that
the latter is counting # of ubos loaded via `ldg` (based on UBO addrs
in push-consts).  But turns out there isn't really any reason to care.
Instead just add an early return in the one code-path that cares about
the number of `ldg` UBOs.

This gets rid of one more thing we need to move from `ir3_shader` to
`ir3_shader_variant`.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5508>