Inlining set_partition_seg_context function.

We used set_partition_seg_context() only before calls to:
 1. update_partition_context()
 2. partition_plane_context()

Moving these functions from vp9_blockd.h to vp9_onyxc_int.h and
inlining  set_partition_seg_context into them. After that it is not
necessary to have {above, left}_seg_context fields in MACROBLOCKD struture,
so removing them also.

Change-Id: I4723f59e1c8f3788432b7f51185d8d747b3a97f9