radeonsi: factor si_query_buffer logic out of si_query_hw

radeonsi: factor si_query_buffer logic out of si_query_hw

This is a move towards using composition instead of inheritance for
different query types.

This change weakens out-of-memory error reporting somewhat, though this
should be acceptable since we didn't consistently report such errors in
the first place.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>