drm/amd/display: Fix unused variable warning

[Why]
It will build failed with unused variable 'dc' with
'-Werror=unused-variable'enabled when CONFIG_DRM_AMD_DC_DCN
is not defined.

Signed-off-by: Tim Huang <tim.huang@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Huang Rui <ray.huang@amd.com>
Reviewed-by: Aaron Liu <aaron.liu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>