drm/ttm: move ttm_tt defines into ttm_tt.h

Let's stop mangling everything in a single header and create one header
per object instead.

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