drm/amdkfd: Add a message when SW scheduler is used

SW scheduler is previously called non HW scheduler, or non HWS. This
message is useful when triaging issues from dmesg.

Signed-off-by: Yong Zhao <Yong.Zhao@amd.com>
Acked-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>