(In reply to comment #32)
> I tested with 3.15-git , and it looks like the kernel parameter has no
> effect there - the same problem occurs again! Even with acpi_sleep=s3_bios ,
> on suspend and resume the laptop LVDS display is blank.
> 
> Does anyone know if this was an intentional change, to remove the ability to
> do this?

Hi, did you check using echo mem > /sys/power/state?

Because e.g. pm-suspend resets those parameters via
/proc/sys/kernel/acpi_video_flags. (Value should be 1, 2, or 3 if you have
kernel parameters set)

For pm-suspend to work (e.g. *Ubuntu) you need to create a file 
/etc/pm/config.d/radeon
which contains
QUIRK_S3_BIOS="true"
QUIRK_S3_MODE="true"