drm/ttm: Use only DRM_MM_SEARCH_BELOW for TTM_PL_FLAG_TOPDOWN

DRM_MM_SEARCH_BEST gets the smallest hole which can fit the BO. That seems
against the idea of TTM_PL_FLAG_TOPDOWN:

* The smallest hole may be in the overall bottom of the area
* If the hole isn't much larger than the BO, it doesn't make much
  difference whether the BO is placed at the bottom or at the top of the
  hole

Reviewed-by: Lauri Kasanen <cand@gmx.com>
Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>