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

Need to guard some things with CONFIG_DRM_AMD_DC_DCN.

Fixes: 707021dc ("drm/amd/display: Enable dpia in dmub only for DCN31 B0")
Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>