drm/amdgpu: fix SI handling in amdgpu_device_asic_has_dc_support()

Properly handle SI DC support when CONFIG_DRM_AMD_DC_SI is not
set.

Fixes: f7f12b25 ("drm/amdgpu: default to true in amdgpu_device_asic_has_dc_support")
Reviewed-by: Evan Quan <evan.quan@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>