Ok I just tried it and it worked on Dell Latitude C610 without redoing the kernel. The modeset option is passed to kernel without redoing initrd. Will amend the post on my site.
T6 can You confirm it?
After following all the steps (without doing the mkinitrd part) reboot and when back to the system run this command as root.
Output should be 1.
[root@eleanor andrzejl]# cat /sys/module/radeon/parameters/modeset
also run glxgears as user
[andrzejl@wishmasbell ~]$ glxgears
Running synchronized to the vertical refresh. The framerate should be
approximately the same as the monitor refresh rate.
299 frames in 5.0 seconds = 59.674 FPS
300 frames in 5.0 seconds = 59.960 FPS
298 frames in 5.0 seconds = 59.576 FPS
The output should be equal (or very close to) Your screen's refresh rate. In my case it's 60 Hz so I am getting about 59 FPS.
If all is good try using some 3D game or app and let us know how it went.