VP9 SVC: fix crash on scaling partition.

VP9 SVC: fix crash on scaling partition.

When scaling up partition from lower resolution layer L, mi_row and
mi_col from L must be smaller than mi_rows and mi_cols from L.

Before this change, the condition was based on mi_rows from top layer
divided by 2, which is not necessarily equal to the mi_rows from lower
resolution layer.

Added variable in SVC structure to keep track of mi_rows and mi_cols
from each spatial layer.

Re-enable partition scaling for 1080p.

BUG=webm:1578

Change-Id: Icc1c701b095cfe0a92bfecca1ed39dbe21da12b6