drm/amdgpu: keep amdgpu_ctx_mgr in ctx structure

Keep amdgpu_ctx_mgr in ctx structure to track fpriv.

Signed-off-by: James Zhu <James.Zhu@amd.com>
Acked-by: Lijo Lazar <lijo.lazar@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>