drm/ttm: fix the fix for huge compound pages

We don't use compound pages at the moment. Take this into account when
freeing them.

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