drm/radeon: Add module param to control SI support

If AMDGPU supports SI, add a module parameter to control SI
support in radeon. It's on by default in radeon, while it will be
off by default in AMDGPU as long as SI support is experimental.

Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Acked-by: Michel Dänzer <michel.daenzer@amd.com>