when using DMIX for ALSA playback, ALSA would not update write index
into driver but maintain it in ALSA LIB level. However, HDMI driver
need write index for HDMI header packing. In this case, HDMI driver
would fail to do HDMI header pack and user hear no sound when using DMIX
in HDMI audio playback.

In this patch, use read index(hw_ptr) for HDMI header packing.

Signed-off-by: Chen Liangjun <b36089@freescale.com>