The pointer in vp8 postproc refers to show_frame_mi which is only
updated on show frame. However, when there is a no-show frame which also
changes the size (thus new frame buffers allocated), show_frame_mi is
not updated with new frame buffer memory.

Change the pointer in postproc to mi which is always updated.

Bug: 842265
Change-Id: I33874f2112b39f74562cba528432b5f239e6a7bd
(cherry picked from commit 52add589)