hal: Changes Relating to DisplayPort Audio

- Update the way we scan for HDMI fb nodes. We
  currently scan nodes 0, 1 & 2 which is not
  sufficient when fb3 is present. We will now open
  fbx until we get an error to ensure we will not
  miss any nodes if new ones are added.

- Fix return value of read_hdmi_sink_caps(), we are
  currently returning the displaytype. The caller
  of the function is interpreting it as an error.

Change-Id: I4a81e93d246d234b86a81eb6597cf641e1ea1991
CRs-Fixed: 1075152