I am having the same exact problem, have A4-7300 with the same exact integrated graphics. The Framebuffer varies depending on the monitor size I plug into it, so smaller resolution gets smaller Framebuffer. 1080p has minimum of 7MB.
As far as Im aware, there is no .kext, nor a way of injecting DSDT with it, as it doesn't show up on in the IORegistryExplorer - there is PCI0 device, but no GFX0 device - as it should be.
I tried forcing it through Chameleon Wizard into the .boot file, but with no luck. Tried about 15 different ATIconfig personalities, but with no difference.
Did you manage to get it going, or are u still stuck on that?