mesa: use strdup() instead of _mesa_strdup()

mesa: use strdup() instead of _mesa_strdup()

We were already using strdup() in various places in Mesa.  Get rid
of the _mesa_strdup() wrapper.  All the callers pass a non-NULL
argument so the NULL check isn't needed either.

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>