drm/amd/powerplay: do not set dpm_enabled flag before VCN/DCN DPM is workable

This dpm_enabled flag will be recognized as the VCN DPM enabled as well. In fact
VCN/DCN DPM on Navi10 is not good so far, so we cannot enable it for now.

Signed-off-by: Huang Rui <ray.huang@amd.com>
Acked-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>