drm/amdgpu: add disable_cu parameter

This parameter will allow disabling individual CUs on module load, e.g.
amdgpu.disable_cu=2.0.3,2.0.4 to disable CUs 3 and 4 of SE2.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Nicolai Hähnle <nicolai.haehnle@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>