vp8:fix threading issues

1 - stops de allocating before threads are closed.
2 - limits threads to mb_rows when mb_rows < partitions

BUG=webm:851
Bug: 30436808

Change-Id: Ie017818ed28103ca9d26d57087f31361b642e09b
(cherry picked from commit 70cca742efa20617c70c3209aa614a70f282f90e)

static void remove_decompressor(VP8D_COMP *pbi)
{
#if CONFIG_ERROR_CONCEALMENT
    vp8_de_alloc_overlap_lists(pbi);
#endif
    vp8_remove_common(&pbi->common);
    vpx_free(pbi);
}

static void remove_decompressor(VP8D_COMP *pbi)
{
#if CONFIG_ERROR_CONCEALMENT
    vp8_de_alloc_overlap_lists(pbi);
#endif
    vp8_remove_common(&pbi->common);
    vpx_free(pbi);
}
