drm/amd/display: Add additional config guards for DCN

[Why&How]

Fix build error by protecting code with config guard
to enable building amdgpu without CONFIG_DRM_AMD_DC_DCN
enabled. This option is disabled by default for allmodconfig.

Signed-off-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Reviewed-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>