drm/amd/display: Update number of dcn21 audio endpoints

[WHY]
Number of audio endpoints wasn't updated from dcn20's 6 when created

[HOW]
Changed num_audio to 4 to match the correct sbios value

Signed-off-by: Michael Strauss <michael.strauss@amd.com>
Reviewed-by: Yongqiang Sun <yongqiang.sun@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>