drm/amdgpu: Use dpm_enabled as dpm state flag

driver will set dpm_enabled to true only when
module parameter amdgpu_dpm not equal to 0 and
smu hw initialize successfully.

Reviewed-by: Evan Quan <evan.quan@amd.com>
Reviewed-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Rex Zhu <Rex.Zhu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>