drm/mm: remove invalid entry based optimization

When the current entry is rejected as candidate for the search
it does not mean that we can abort the subtree search.

It is perfectly possible that only the alignment, but not the
size is the reason for the rejection.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Nirmoy Das <nirmoy.das@amd.com>
Link: https://patchwork.freedesktop.org/patch/369394/