drm/amdgpu: stop trying to schedule() with a spin held

Drop the lock before calling cancel_delayed_work_sync().

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=96445



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