gallium: make get_query_result return union* and not void*

gallium: make get_query_result return union* and not void*

This replaces the cryptic void* parameter with a union.
(based on union r600_query_result)

Users of this can still pass uint64* in it, but that cannot work for every
query type, obviously. Most importantly, the code now documents what should
be expected from get_query_result.

This also adds pipe_query_data_pipeline_statistics as per the D3D11 docs.

v2: fix indentation, add comments and use the doxygen style

Reviewed-by: Brian Paul <brianp@vmware.com>