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>

drivers/gpu/drm/amd/amdgpu/amdgpu_ctx.c

@@ -613,7 +613,8 @@ int amdgpu_ctx_ioctl(struct drm_device *dev, void *data,
 		if (args->in.flags)
 			return -EINVAL;
 		r = amdgpu_ctx_stable_pstate(adev, fpriv, id, false, &stable_pstate);
-		args->out.pstate.flags = stable_pstate;
+		if (!r)
+			args->out.pstate.flags = stable_pstate;
 		break;
 	case AMDGPU_CTX_OP_SET_STABLE_PSTATE:
 		if (args->in.flags & ~AMDGPU_CTX_STABLE_PSTATE_FLAGS_MASK)