The device to use those ioctls on is actually the ti81xxhdmi device, not the V4L2 device. You will need to open it separately - it's named something like "/dev/TI81XX_HDMI".
The EDID from the HDMI output is also accessible by reading /sys/devices/platform/vpss/display0/edid (as a text file - it's in a human-readable table of hex bytes), but the HPD status can't be found by this method.
- Mark