drm/amdkfd: Update KFD TTM mem limit

Use the helper function in TTM to get TTM memory
limit and set KFD's internal mem limit. This ensures
that KFD's TTM mem limit and actual TTM mem limit are
exactly same.

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