audio/hal: Configure HDMI channels based on output stream channels

- Current HAL configures HDMI channels based on sink capabilities,
  even when the output content is stereo. DSP upmixes the content
  if the HDMI backend is configured for 5.1 channels.
- This change ensures that HDMI backend is configured based on
  output stream channels.

Bug: 7290997.
Change-Id: I42b2773b8f4ccc62203c13ff9ac6a6511df0705f