drm/amdgpu: fix vega10 graphic hang issue in S3 test

mmVGT_INDEX_TYPE has no default value, need to make sure
it's initialized when gfx is initialized.

Signed-off-by: Ken Wang <Ken.Wang@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>