drm/amdgpu: add judgement for dc support

Drop DC initialization when DCN is harvested in VBIOS. The way
doesn't affect virtual display ip initialization.

Signed-off-by: Likun Gao <Likun.Gao@amd.com>
Signed-off-by: Asher Song <Asher.Song@amd.com>
Reviewed-by: Guchun Chen <guchun.chen@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>