gallium/hud: Fix support for PIPE_DRIVER_QUERY_TYPE_FLOAT

gallium/hud: Fix support for PIPE_DRIVER_QUERY_TYPE_FLOAT

Evidently, nobody has used PIPE_DRIVER_QUERY_TYPE_FLOAT up to this
point.  Adding a driver query of this type which returns the query
value in pipe_query_result::f resulted in garbage output in the HUD.

The problem is the pipe_query_result::f field was being accessed as
through the u64 field and being added to the query_info::results_cumulative
field.  This patch checks for PIPE_DRIVER_QUERY_TYPE_FLOAT in a few
places and scales the float by 1000 before converting to uint64_t.

Also, add some comments to explain the query_info::result_index field.

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