Try updating your APU driver from AMD Support. You can download the latest from here: APU . Also see if you can update the AMD Chipset. You can download it from the same page at the apu driver under "Optional Downloads".
@BlackZion mentioned about using the BIOS. Here is a PDF Link to your Manual for your Motherboard : http://asrock.pc.cdn.bitgravity.com/Manual/FM2A68M-DG3+.pdf .
This is the only part of the BIOS that seems to relate to GPU Memory from the link above:
Adjust the VRAM size in BIOS.