drm/amdgpu: move scheduler wqueue handling into callbacks

Move the scheduler wqueue stopping and starting into
the ring reset callbacks.  On some IPs we have to reset
an engine which may have multiple queues.  Move the wqueue
handling into the backend so we can handle them as needed
based on the type of reset available.

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