drm/amdgpu: enable DCN for navi10 headless SKU

There is a NULL pointer crash when DCN disabled on headless SKU.
On normal SKU, the variable adev->ddev.mode_config.funcs is
initialized in dm_hw_init(), and it is fine to access it in
amdgpu_device_resume(). But on headless SKU, DCN is disabled,
the funcs variable is not initialized, then crash arises.
Enable DCN to fix this issue.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Guchun Chen <guchun.chen@amd.com>
Signed-off-by: Tianci.Yin <tianci.yin@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>