asoc: hdmi: return 0 when get_audio_edid_blk is NULL

when get_audio_edid_blk is NULL, msm_ext_disp_edid_ctl_info
returns error value. And it would cause tinymix doesn't have
full mix cmd list. Return 0 instead to avoid such issue.

Change-Id: Ic13067483b373fa4a81b663ece3d3e4a18ea8a87
Signed-off-by: Meng Wang <mengw@codeaurora.org>