drm/amd/pp: Fix gfx ring test failed on Fiji without hw avfs support

caused by
'commit ca82cec868d1 ("drm/amd/pp: Simplified the avfs btc state on smu7")'

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Rex Zhu <Rex.Zhu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>