Simply switching to ARM, will not suffice AMD to catch up with Qualcomm/Apple in power efficiency. Apple/Qualcomm has advantages other than the use of the ARM ISA itself.
1. Custom CPU core designs.
Both Apple and Qualcomm have custom ARM core designs, which as of now, are better than the stock ARM cores AMD/Nvidia will have to use. The next generation of ARM cores (X5/A725) might change that though.
2. A legacy in mobile computing.
Both Apple and Qualcomm have been designing smartphone SoCs for more than a decade. Thus, they have mastered the art of power efficiency, with many tricks and solutions- especially on low-power and idle efficiency. AMD/Nvidia don’t have this experience in making smartphone SoCs, and thus lack the knowledge/experience that comes with it.