drm/amdgpu: Add module param to control SI support

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

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>

[ Michel Dänzer: Squash in amdgpu_si_support initialization fix ]
Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>