vp9_ethread: move max/min partition size to mb struct

vp9_ethread: move max/min partition size to mb struct

The max_partition_size and max_partition_size are set at the
beginning while setting speed features, and then adjusted at
SB level. Moving them to mb struct ensures there is a local
copy for each thread.

Change-Id: I7dd08dc918d9f772fcd718bbd6533e0787720ad4