drm/amdgpu: check return status before using stable_pstate

Clang static analysis reports this problem
amdgpu_ctx.c:616:26: warning: Assigned value is garbage
  or undefined
  args->out.pstate.flags = stable_pstate;
                         ^ ~~~~~~~~~~~~~
amdgpu_ctx_stable_pstate can fail without setting
stable_pstate.  So check.

Fixes: 8cda7a4f ("drm/amdgpu/UAPI: add new CTX OP to get/set stable pstates")
Signed-off-by: Tom Rix <trix@redhat.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>