it is not clear to me how HDMI resolution/modeset works on imx6. There are involved the HDMI supported modesets and the monitor supported modeset.
1) afaik, there is a table (33-3) that shows only certain videomode supoprted (1920 x 1080, 1280x720 and lower). But for example initializing display from u-boot and stopping at U.Boot prompt, seems also different resolution are possible, like if they are forced, they work.
2) also, Linux seems to be able so sync to hdmi 1024x768 (mxc_sdc_fb fb.29: 1024x768 h_sync,r,l: 136,24,160 v_sync,l,u: 6,3,29 pixclock=65002000 Hz), and this is also strange since seems to be not supported.
Every clarification would be helpful