drm/amdgpu: give warning before sleep in kiq_r/wreg

to catch error that may schedule in atomic context early on

Signed-off-by: Monk Liu <Monk.Liu@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>