drm/radeon: possible buffer overflow

Buffer 'afmt_status' of size 6 could overflow, since index 'afmt_idx' is
checked after access.

Fixes: 5cc4e5fc

 ("drm/radeon: Cleanup HDMI audio interrupt handling for evergreen")
Co-developed-by: Ivanov Mikhail <ivanov.mikhail1@huawei-partners.com>
Signed-off-by: Konstantin Meskhidze <konstantin.meskhidze@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>