drm/amdgpu: use fence_context to judge ctx switch v2

Use of the ctx pointer is not safe, because they are likely already
be assigned to another ctx when doing comparing.

v2: recreate from scratch, avoid all unnecessary changes.

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