drm/amd/powerplay: set dpm_enabled flag but don't enable vcn dpm

This patch sets dpm_enabled flag but don't enable vcn dpm, because vcn dpm
doesn't work so far and we needs to enable the sysfs interfaces.

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>