r600g: rework queries

r600g: rework queries

We always mapped the query buffer in begin_query, causing stalls
if the buffer was busy.

This commit reworks it such that the query buffer is only mapped
in get_query_result as it's supposed to be.

The query buffer is no longer treated as a ring buffer. Instead, the results
are just appended and when the buffer is full, we create a new one. One query
can have more than one query buffer, though that's a very rare case.
Begin_query releases all query buffers.

Reviewed-by: Jerome Glisse <jglisse@redhat.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>