Fix a bug when combining new-quant + supertx

Previously, we assumed that av1_init_plane_quantizers is always called with
segment_id == xd->mi[0]->mbmi.segment_id (and use the latter to derive the value
of 'qindex' to use in the quantizer). But this is no longer true when supertx
is enabled. This patch instead remembers the value of 'qindex' derived from
the latest call to av1_init_plane_quantizers and uses that directly.

Change-Id: Ifa1c5bf74cad29942ff79b88ca92c231bc07f336