drm/amdgpu/display: fix warnings when CONFIG_DRM_AMD_DC_DCN is not set

Properly protect the relevant code with CONFIG_DRM_AMD_DC_DCN.

Fixes: 0b08c54b ("drm/amd/display: Fix the display corruption issue on Navi10")
Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>