Sort variables dependency in read_uncompressed_header

Remove a few repeated data structure reads from
read_uncompressed_header.

Change-Id: I6eb741b39f9415ad0aa4631dfbf4a1ace4eba56a

vp9/decoder/vp9_decodeframe.c

diff --git a/vp9/decoder/vp9_decodeframe.c b/vp9/decoder/vp9_decodeframe.c
index eb9b797..642b158 100644
--- a/vp9/decoder/vp9_decodeframe.c
+++ b/vp9/decoder/vp9_decodeframe.c
@@ -1311,8 +1311,8 @@
 static size_t read_uncompressed_header(VP9Decoder *pbi,
                                        struct vp9_read_bit_buffer *rb) {
   VP9_COMMON *const cm = &pbi->common;
-  RefCntBuffer *const frame_bufs = cm->buffer_pool->frame_bufs;
-  BufferPool *const pool = pbi->common.buffer_pool;
+  BufferPool *const pool = cm->buffer_pool;
+  RefCntBuffer *const frame_bufs = pool->frame_bufs;
   int i, mask, ref_index = 0;
   size_t sz;