I have a similar issue with a samsumg series 3 laptop with "VGA compatible
controller: Advanced Micro Devices, Inc. [AMD/ATI] BeaverCreek [Radeon HD
6620G]" and "VGA compatible controller: Advanced Micro Devices, Inc. [AMD/ATI]
Seymour [Radeon HD 6400M/7400M Series]". after resume an external monitor show
the system is still responsive.

BUT I can make the screen turn on after suspend with the next command:
sudo pm-suspend --quirk-test --quirk-s3-bios

so if you can try and this work maybe we can kick this issue.