Intel's Broadwell is now faster than any AMD APU in both CPU and GPU. Intel has come a very long way in integrated graphics and they have all but erased AMD from every market segment from top to bottom. Starting with Skylake, AMD will no longer be competitive anywhere in either CPU or GPU from top to bottom of the x86 product stack. This is the situation right now, today. Broadwell's GPU is
scary good. Scary for AMD anyways.
http://www.anandtech.com/show/9320/intel-broadwell-review-i7-5775c-i5-5675c/7
If it really came down to that scenario, I'm sure by the time the PS5 is due to debut, Sony could rely on Intel to deliver a console x86 SoC that was competitive in both CPU and GPU with anything AMD could put forward. Remember that Intel is something like 5 years ahead of AMD in process nodes at this point, and they have been tick-tocking along during those 5 years as well. Skylake will mark the 3rd new architecture since Bulldozer debuted on the AMD side, Intel has been
busy.
Of course if Sony cared more about GPU power than CPU, there's always Nvidia Tegra or Denver. That seems fairly unlikely though, as you say, Sony is likely keen to stay on x86 and so are MS. Nintendo is likely to move to x86 with Project NX as well, PowerPC is a dead end and I'm just not seeing Nintendo betting on ARM at this point.