gallium/radeon: add VRAM-vis-usage HUD query

This new query returns the current visible usage of VRAM accessed
by the CPU. It will return 0 on radeon because it's unimplemented.

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Tested-by: Edmondo Tommasina <edmondo.tommasina@gmail.com>
Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
5 files changed