drm/amdgpu: save the setting of VM_CONTEXT_CNTL

MES firmware needs the setting of VM_CONTEXT_CNTL to perform
vmid switch. Save the initial setting when hub initializing.

Signed-off-by: Jack Xiao <Jack.Xiao@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>