drm/amdgpu: add amdgpu_error_* debugfs file

This allows us to insert some error codes into the bottom of the pipeline
on an engine.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Luben Tuikov <luben.tuikov@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>