st/mesa: improve shader cache debug info

st/mesa: improve shader cache debug info

This will explicitly state that we are following the fallback
path when we find invalid/corrupt cache items. It will also
output the fallback message when the fallback path is forced
via an environment variable, the following patches will allow
this.

Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>