drm/amd/amdgpu: Use IP discovery data to determine VCN enablement instead of MMSCH

In the past, we use MMSCH to determine whether a VCN is enabled or not.
This is not reliable since after a FLR, MMSCH may report junk data.

It is better to use IP discovery data.

Signed-off-by: Bokun Zhang <Bokun.Zhang@amd.com>
Signed-off-by: Peng Ju Zhou <PengJu.Zhou@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>