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: 41724ea2 ("drm/amd/display: Add DP 2.0 MST DM Support")
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: Lyude Paul <lyude@redhat.com>
Cc: Dave Airlie <airlied@gmail.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211027223914.1776061-1-alexander.deucher@amd.com