drm/radeon/hdmi: store info about all AFMT blocks

Introduce special struct radeon_afmt for this purpose.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>