drm/ttm: Add DRM_MM_SEARCH_BELOW for TTM_PL_FLAG_TOPDOWN

If the BO should be placed at the top of the area, we should start looking
for holes from the top.

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>