drm/amdgpu: add a message to indicate when powerplay is enabled (v2)

Makes it clear to the user which power management path is in
use.

v2: make consistent with dpm

Reviewed-by: Rex Zhu <Rex.Zhu@amd.com>
Reviewed-by: Tom St Denis <tom.stdenis@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>