drm/amdgpu: add amdgpu.cg_mask and amdgpu.pg_mask parameters

They allow disabling clock and power gating from the kernel command line,
which hopefully helps with diagnosing problems in the field.

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>