vp9_dthread: pass frame counts to decoder functions

The current multi-threaded tile decoder requires that the videoes
are encoded with frame_parallel_decoding_mode = 1. This requirement
is not necessary, and is better to be removed. This patch includes
the first part of the work.

Change-Id: Ic7695fb3cfe13f9022582c9f0edd2aa6e2e36d28
5 files changed