Hook up VP8D_GET_LAST_REF_USED

Commit 892e23a5 introduced support for the VP8D_GET_LAST_REF_USED,
but missed the mapping of the control id to the underlying function,
so it was unavailable to applications.

In addition, the underlying function vp8_references_buffer() is
moved from common/postproc.c to decoder/onyxd_if.c as postproc.c is
not built in all configurations.

Change-Id: I426dd254e7e6c4c061b70d729b69a6c384ebbe44