drm/amdgpu: stop touching sched.ready in the backend

This unfortunately comes up in regular intervals and breaks
GPU reset for the engine in question.

The sched.ready flag controls if an engine can't get working
during hw_init, but should never be set to false during hw_fini.

v2: squash in unused variable fix (Alex)

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