Is using AES-256 with a key of less than 256 bits of entropy still quantum-resistant?
Currently, the quantum attacks work on the block cipher itself. The Grover's search algorithm reduces the complexity of 256-bit key into 128-bit since it has complexity $\mathcal{O}(\sqrt{n})$ and in the case of 256-bit $\mathcal{O}(\sqrt{2^{256}}) = \mathcal{O}(2^{128})$

Since you have started with 128-bit entropy, theoretically, the quantum attacker to block cipher itself with a 256-bit key will not benefit from Grover's algorithm since Grover's algorithm will search 256-bit. Keep in mind that 256-bit space is infeasible for the quantum. 

What if in an unforeseeable future, someone builds a quantum circuit for the PBKDF2 + AES combination that is the search for the 128-bit input to PBKDF2 and uses the output for the AES-256 key and perform Grover's kind of attack? 

This can be done by extending the AES-256 Grover Search with a 128-Q-bit PBKDF2 Quantum circuit. Since the input is 128-bit, the search will be $\mathcal{O}(2^{64})$ to find the ciphertext.

The attacker, always, will use the weakest point!.