Comment 7 for bug 915408

The problem is the modeline from your monitor for the 1920x1080 mode is greater than 165 Mhz which is the current cut off for single link mode in the driver:
(II) RADEON(0): Modeline "1920x1080"x60.0 172.78 1920 2040 2248 2576 1080 1081 1084 1118 -hsync +vsync (67.1 kHz)

It should work if you specify a modeline with a lower clock E.g.,
Modeline "1920x1080R" 138.50 1920 1968 2000 2080 1080 1083 1088 1111 +hsync -vsync