drm: move "struct drm_vma_entry" to drm_vm.c

Make all the drm_vma_entry handling local to drm_vm.c and hide it from
global headers. This requires to extract the inlined legacy drm_vma_entry
cleanup into a small helper and also move a weirdly placed drm_vma_info
helper into drm_vm.c.

Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Reviewed-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>