Reduce the memset call in tile decoding

Reduce the memset call in tile decoding

When the error resilient mode is on, the decoder resets mode info structure
to zero once per frame. This makes decoder about 10x slower if we decode
a single tile at a time. This patch resolves the issue by only memset mode
info of those decoded tiles. Currently, to decode a frame, tile decoding is
less than 2x slower than frame decoding.

Change-Id: Ia3fd88d91a4e74e7bbbc6547d87c24d085a1533e