If you're getting a max resolution of 1280x720 on a monitor that supports 1920x1080, then you're still using the lite version of the driver that came with the OS (i.e., you haven't even installed v1.x/2.x yet).
You can double-check the version by entering the following in a shell window:
version RadeonHD.chip
If it comes back with a version 0.x, then you're using the lite driver.
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