drm/amd/amdgpu: Update VM function pointer

When VM state changed and system in large bar mode,
make sure to use CPU update function, otherwise use
SDMA function.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Gang Ba <gaba@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>