vp9: fix ubsan sub-overflows
vp9: fix ubsan sub-overflows

Fix errors reported by UBSan diagnostics:
1. /vp9/encoder/vp9_pickmode.c:308:29: unsigned integer overflow:
   99 - 100 cannot be represented in type 'unsigned int'
2. /vp9/encoder/vp9_pickmode.c:330:27: unsigned integer overflow:
   21976 - 21978 cannot be represented in type 'unsigned int'
3. /vp9/encoder/vp9_pickmode.c:468:13: unsigned integer overflow:
   18852144 - 18852149 cannot be represented in type 'unsigned int'

(Notice that line numbers might vary a bit because fixes have
been applied incrementally i.e. fix for error #1 affects line
number reported in #2)

Fix by calculating difference instead of wrapping around to
a value near maximum.

Test: Cuttlefish webrtc with VP9 codec
Change-Id: I4f85712028647e915a4e2da31e4b0a266e9e2705

webmproject/libvpx,webmproject/libvpx,webmproject/libvpx,webmproject/libvpx,webmproject/libvpx,webmproject/libvpx