Properly set size based on actual buffer layout

VP9FrameSizeTestsLarge.OneByOneVideo has been causing a failure in
jenkins libvpx__unit_tests-valgrind_long for "using of uninitialized
memory", the root cause was that the input image for this test was
not initialized with proper size, therefore plan U and V were not
initialized at all.

This commit fixes the size initialization, and resolves the issue.

Change-Id: Ic4dd1542b7bb0cb260a1e0aeeb505db21ae5edc8