drm/amd/display: fix build error on arm64

dcn20_build_mapped_resource() and dcn20_acquire_dsc() is not defined,
if CONFIG_DRM_AMD_DC_DCN is disabled.

Fix the following build error on arm64:

  ERROR: modpost: "dcn20_build_mapped_resource" [drivers/gpu/drm/amd/amdgpu/amdgpu.ko] undefined!
  ERROR: modpost: "dcn20_acquire_dsc" [drivers/gpu/drm/amd/amdgpu/amdgpu.ko] undefined!

Fixes: 20dad381 ("drm/amd/display: Add a helper to map ODM/MPC/Multi-Plane resources")
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>