amdgpu: move ttm stuff to amdgpu_ttm.h

Signed-off-by: Flora Cui <Flora.Cui@amd.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>