Simplify aom_get_qmlevel

Change the way the qmlevel is calculated from min_qmlevel,
max_qmlevel and qindex. This fixes an issue where we could
end up with qmlevel = max_qmlevel + 1 - see the associated
bug report for details.

BUG=aomedia:747

Change-Id: Iaa5884d37b7dfbfd703c94449597bd0050525168

av1/common/quant_common.h

diff --git a/av1/common/quant_common.h b/av1/common/quant_common.h
index 2795425..92843fe 100644
--- a/av1/common/quant_common.h
+++ b/av1/common/quant_common.h
@@ -48,9 +48,7 @@
 // Reduce the large number of quantizers to a smaller number of levels for which
 // different matrices may be defined
 static INLINE int aom_get_qmlevel(int qindex, int first, int last) {
-  int qmlevel = (qindex * (last + 1 - first) + QINDEX_RANGE / 2) / QINDEX_RANGE;
-  qmlevel = AOMMIN(qmlevel + first, NUM_QM_LEVELS - 1);
-  return qmlevel;
+  return first + (qindex * (last + 1 - first)) / QINDEX_RANGE;
 }
 void aom_qm_init(struct AV1Common *cm);
 qm_val_t *aom_iqmatrix(struct AV1Common *cm, int qindex, int comp,