This subject has caught my interest for a while, and this thread always pops up in the google results. I've done a lot of research about it, even more since I started grabbing lots of 10-bit HEVC encodes. My experience is that most of the current high end chipsets have support for it (maybe all of them), so it shouldn't be a problem nowadays. Of course, it wasn't like this when this thread started in 2014.
Now the real deal is finding support in the mid range chipsets. I've come accross two wikipedia pages that are relatively kept up to date and I've been contributing myself.
For Snapdragon chipsets
en.wikipedia.org
For Mali GPUs (Mediatek chipsets)
en.wikipedia.org
So you can have an idea of the current HW codec suport for these chipsets. About Exynos, none of the 7xxx series have support for 10 bit HEVC as far as I know, even though some of them do support VP9. I guess you have to go to the high end range for that.
Something I've noticed as well is that you can find reviews for the device you want, specially in GSMarena, and if the camera supports 4k video recording by default, it is pretty much a sure thing that the device has 10 bit HEVC support, though I can't really confirm this.
Hope this info can be useful for someone later on. This thread has a lot of info but is very spread.