Check if the requested stable pstate matches the current one before
changing it.  This avoids changing the stable pstate on context
destroy if the user never changed it in the first place via the
IOCTL.

v2: compare the current and requested rather than setting a flag (Lijo)

Fixes: 8cda7a4f ("drm/amdgpu/UAPI: add new CTX OP to get/set stable pstates")
Reviewed-by: Lijo Lazar <lijo.lazar@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>