drm/amdgpu: enable iceland powerplay manually

It's able to enable iceland powerplay manually via the module
parameter. The default state is disabled.

Signed-off-by: Huang Rui <ray.huang@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>