drm/ttm: Helper function to get TTM mem limit

Add a helper function to get TTM memory limit. This is
needed by KFD to set its own internal memory limits.

Signed-off-by: Mukul Joshi <mukul.joshi@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>