drm/amdgpu: restore original stable pstate on ctx fini

Save the original stable pstate on ctx init and restore
it on ctx fini so that we restore a manually selected
stable pstate on ctx exit.

v2: fix init order (Alex)
v3: don't add new variable to ctx struct (Evan)

Fixes: c65b364c ("drm/amdgpu/ctx: only reset stable pstate if the user changed it (v2)")
Reviewed-by: Evan Quan <evan.quan@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>