drm/amdgpu/pm: bail on sysfs/debugfs queries during platform suspend

The GPU is in the process of being shutdown.  Spurious queries during
suspend and resume can put the SMU into a bad state.  Runtime PM is
handled dynamically so we check if we are in non-runtime suspend.
Reviewed-by: Evan Quan <evan.quan@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>