drm/radeon: Make si_support and cik_support parameters always available

This will allow amdgpu-pro / other out-of-tree amdgpu builds to make use
of these options for using the out-of-tree amdgpu driver instead of the
in-tree radeon driver in a clean way.

Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>