Consider the utility maximisation problem:
$$
\max_{x_1, x_2} u(x_1, x_2) \text{ s.t. } p_1 x_1 + p_2 x_2 \le m.
$$
If we multiply all prices and income by the same number $t > 0$, we obtain the problem:
$$
\max_{x_1, x_2} u(x_1, x_2) \text{ s.t.} t p_1 x_1 + t p_2 x_2 \le t m
$$
However if we divide the left and right hand side of the constraint by $t$ we get the original problem back.
In other words, the two problems are identical. This also means that the two problems will also give exactly the same solution.
Summarizing, all demand functions are homogeneous of degree zero whatever the utility function because multiplying all prices and income by the same positive number does not change the problem: everything is $t$-times more expensive but you also have $t$-times more income.