drm/amd/display: fix error case handling

Otherwise, we will run into error case path.

v2: fix build when CONFIG_DRM_AMD_DC_DCN is not set

Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Guchun Chen <guchun.chen@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>