Currently, fetching the partial results (VK_QUERY_RESULT_PARTIAL_BIT)
of an unavailable occlusion query via vkGetQueryPoolResults can
return invalid values. anv returns slot.end - slot.begin, but in the
case of unavailable queries, slot.end is still at the initial value
of 0. If slot.begin is non-zero, the occlusion count underflows to
a value that is likely outside the acceptable range of the partial
result.

This commit fixes vkGetQueryPoolResults by always returning 0 if the
query is unavailable and the VK_QUERY_RESULT_PARTIAL_BIT is set.

Cc: <mesa-stable@lists.freedesktop.org>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Part-of: <mesa/mesa!3586>