drm/amdgpu/pm: clarify debugfs pm output

On APUs power is SoC power, not just GPU.
Clarify that for UVD/VCE/VCN the IP is powered down,
not disabled which can confusing and lead to concerns
that the IP is actually not available.

Reviewed-by: Yang Wang <kevinyang.wang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>