Here's a patch sequence which makes my PCH-connected eDP panel work. The main bug was a pile of places where the driver was incorrectly treating a PCH connected eDP panel like a CPU connected eDP panel, setting incorrect bits in the DP_CTL register and failing to configure the TRANS_DP_CTL register entirely.
Beyond that, this eDP panel appears very sensitive to panel power sequencing, and I found a bunch of minor errors there. I switched from using blind timings to polling the panel power sequencing status register to make sure we waited until that thought things were done, and so that any panel power sequencing errors would show up in the kernel log.
Finally, I noticed that the BIOS tried harder to get the link trained, by simply starting over when it failed and trying the whole sequence up to 5 times. This is not part of the DP spec, but given how bad failing to train a panel is, it seems like it might be a good idea.
The three most interesting patches are the one which handles PCH eDP more like PCH DP, the one which switches to using the panel power sequencing hardware for all delays and finally the patch which tries to do the panel power-up/down in the same order for both dp_prepare/commit and dp_dpms.
All of these patches are on my pch-edp-fixes branch at
git://people.freedesktop.org/~keithp/linux
If you've got a PCH connected eDP display, I'd love to know if this makes it work. If you've got a CPU connected eDP display or PCH connected DP, please see if this causes any problems.
(I've got several machines failing to resume with this patch, but I've checked and it's not the fault of anything in the i915 directory; applying this sequence to v3.1 makes suspend/resume work fine).