drm/ttm: Don't count pages in SG BOs against pages_limit

Pages in SG BOs were not allocated by TTM. So don't count them against
TTM's pages limit.

Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210414064804.29356-9-Felix.Kuehling@amd.com