drm/amdgpu: wrap those atombios APIs used by SI under CONFIG_DRM_AMDGPU_SI

No need to compile them on CONFIG_DRM_AMDGPU_SI disabled.

Signed-off-by: Evan Quan <evan.quan@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>