freedreno/a6xx: Implement primitive count queries on GPU

The driver can't determine PIPE_QUERY_PRIMITIVES_GENERATED or
PIPE_QUERY_PRIMITIVES_EMITTED once we support geometry or
tessellation, since these stages add primitives at runtime.  Use the
WRITE_PRIMITIVE_COUNTS event to write back the primitive counts and
implement a hw query for this.

Reviewed-by: Rob Clark <robdclark@gmail.com>