drm/amdgpu: Skip err_count sysfs creation on VF unsupported RAS blocks

VFs are not able to query error counts for all RAS blocks. Rather than
returning error for queries on these blocks, skip sysfs the creation
all together.

Signed-off-by: Victor Skvortsov <victor.skvortsov@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>