drm/amdgpu: enable amdgpu_dc module parameter

It doesn't work under IP discovery mode. Make it work!

Signed-off-by: Lang Yu <Lang.Yu@amd.com>
Reviewed-by: Alex Deucher <aleander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>