drm/amd/display: avoid to authentication when DEVICE_COUNT=0

[why]
we don't support authentication with DEVICE_COUNT=0

[how]
check value DEVICE_COUNT before doing authentication

Signed-off-by: Yu-ting Shen <Yu-ting.Shen@amd.com>
Reviewed-by: Wenjing Liu <Wenjing.Liu@amd.com>
Acked-by: Wayne Lin <waynelin@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>