[M120-LTS] Fix a bug in simple motion search

This change fixed a bug revealed by b/311294795.
In simple motion search, the reference buffer pointer needs to be
restored after the search. Otherwise, it causes problems while the
reference frame scaling happens. This CL fixes the bug.

Bug: b/311294795
Change-Id: I093722d5888de3cc6a6542de82a6ec9d601f897d
(cherry picked from commit 50ed636e49db2b8fa2436413480f11ab1f2a2d1a)
(cherry picked from commit 75d7727f58ddf751b4333a4df94cab1fd920a35c)

• test/encode_api_test.cc[diff]
• vp9/encoder/vp9_encodeframe.c[diff]