zink: clean up const-value handling for get_ssbo_size

zink: clean up const-value handling for get_ssbo_size

nir_src_as_const_value can return null pointers, and in other places we
at least assert for this. So let's do that here as well, which makes
Coverity a bit less paranoid.

While we're at it, avoid duplicating the nir_src_as_const_value call to
the same source.

CID: 1444291

Reviewed-By: Mike Blumenkrantz <michael.blumenkrantz@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/12594>