drm/amdgpu: fix possible pstate switch race condition

Added lock protection so that the p-state switch will
be guarded to be sequential. Also update the hive
pstate only all device from the hive are in the same
state.

Signed-off-by: Evan Quan <evan.quan@amd.com>
Reviewed-by: Feifei Xu <Feifei.Xu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>