drm/radeon: rework audio detect (v4)

1. Always assign audio function pointers even if the display does
not support audio.  We need to properly disable the audio stream
when when using a non-audio capable monitor.  Fixes purple line
on some hdmi monitors.

2. Check if a pin is in use by another encoder before disabling
it.

v2: make sure we've fetched the edid before checking audio and
    look up the encoder before calling audio_detect since
    connector->encoder may not be assigned yet.  Separate
    pin and afmt.  They are allocated at different times and
    have no dependency on eachother.
v3: fix connector fetching in encoder functions
v4: fix missed dig->pin check in dce6_afmt_write_latency_fields

bugs:
https://bugzilla.kernel.org/show_bug.cgi?id=93701
https://bugzilla.redhat.com/show_bug.cgi?id=1236337
https://bugs.freedesktop.org/show_bug.cgi?id=91041



Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org