There seems to be a number of questions about screen resolutions not being detected by the default open source drivers. Try searching AU for "Undetected Screen Resolution" and you will be able to see them.Here are a few examples:-
- How to increase the default screen resolution without a monitor plugged in
- Low Resolution on Boot, not Login
- Resolution resets after every reboot, Screen Resoultion stuck at 1024x768
- Need to change the resolution options
Therefore I think this answer here may be a canonical "answer" for those type of questions. But does the community think so?