mesa: Move register_allocate.c to util.

mesa: Move register_allocate.c to util.

The r300 gallium driver is using it outside of the Mesa tree, and I wanted
to do so for vc4 as well.  Rather than make the multiple-definitions
problem even more complicated, just move it to more-shared code.

v2: Don't forget to delete the symlink in r300 (review by Matt).
    Delete more r300-helper references (review by Emil)
    Don't prefix util/ header inclusion with "util/" (review by Emil)

Reviewed-by: Matt Turner <mattst88@gmail.com> (v1)
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com> (v1)