r600g: use backend mask for occlusion queries

r600g: use backend mask for occlusion queries

Use backend_map kernel query if supported, otherwise analyze ZPASS_DONE
results to get the mask.

Fixes lockups with predicated rendering due to incorrect query buffer
initialization on some cards.

Note: this is a candidate for the 7.11 branch.

Signed-off-by: Vadim Girlin <vadimgirlin@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>