[965] Fix TTM relocation caching overzealousness.

[965] Fix TTM relocation caching overzealousness.

The failure mode that was a available was:
reloc 1 -> target_buf
exec: PRESUMED_OFFSET wrong, buffer migrates, r1 entry updated.
reloc 2 -> target_buf
exec: suppose buffer migrates again.  PRESUMED_OFFSET wrong. r2 entry updated.
reloc 1 -> target_buf
exec: suppose buffer doesn't migrate.  PRESUMED_OFFSET right. no relocations
      performed.  r1 has stale pointer at original location.

Failures were reported with OGLconform's VBO test and SPECviewperf90, though
I haven't confirmed that this fixes it.