Energy consumption of a small TFT
Mobile phones give quite a good benchmark on display power consumption.

For example a Samsung Galaxy S5 has a 5", 1920x1080 pixels display. And it has a 2800 mAh battery and is able to do a benchmark at 200 nit for around 7.5 hours (based on phonearena). 

So the phone needs an average of 375 mA during that time. I'd guess that most of it goes in the display, so there is not that much current to be saved, but they run on a lower voltage, so you can save some power. (If you calculate pixels per milliamp they are insanely more efficient)

It's quite easy to get your hands on those mobile phone displays, but it's a lot harder to get your hands on an efficient driver to use the displays outside of their normal environment (at least that's my experience). 

I don't know which interface you want to use, but I think that's part of the problem, going from RPi -> HDMI -> Display is more power hungry than say a display which goes µP -> Display (embedded displayport).

The 3.3 V of the RPi is very weak, you won't be able to power a display directly from it. It's limited to 50 mA if I remember correctly.