drm/amdgpu/display: move link_bandwidth_kbps under CONFIG_DRM_AMD_DC_DCN

It's only used when CONFIG_DRM_AMD_DC_DCN is set.  Fixes and set but
not used warning.

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