It has taken Intel Linux developers a while to get Panel Self Refresh (PSR) and its dependencies all figured out, but now it appears they are comfortable with soon enabled it by default to save on power consumption.
Panel Self Refresh has been available for a few years now via the (e)DP DisplayPort 1.3 specification from VESA. Panel Self Refresh is about reducing power consumption when the system is idling / the display is static. The idea behind Panel Self Refresh is to have the display keep using the same frame-buffer whenever the contents are unchanged, which can allow power-savings by shutting down unnecessary GPU circuitry. It's basically skipping over a lot of work for the hardware by using the same (idle) image stored by the display until new updates are sent from the GPU. PSR can be huge for laptop and mobile users if often staring at an unchanged display when reading a web-site or document or the system is simply idling. Reports from different vendors I've seen indicate the PSR power saving benefits can be anywhere from ~29% to 85% based upon the hardware and other factors.
Intel hasn't enabled PSR by default due to issues, but they should soon be doing so for their Linux driver. Rodrigo Vivi commented on the patch to enable PSR by default, "With a reliable frontbuffer tracking and all instability corner cases solved let's re-enabled PSR by default on all supported platforms."
The patch has yet to land, but let's hope that it will be approved and turn out to be fine for getting into DRM-Next for Linux 4.5. I'll work on some PSR idle power consumption tests shortly on Phoronix.