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

[ Upstream commit a2b4785f ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>