Further testing shows large fps drops from 60 to ~0 whenever used GTT exceeds
550mb.  Tested kernels going back to 3.10.63 with no improvement.

Installed kernel 3.19.0-rc3 and llvm-3.4.2 to test mesa releases.

Confirmed this is a regression in mesa.  Mesa-10.0.4, 10.3.5 and 10.3.6 manage
at least 30fps or higher with GTT usage over 550mb.

Tested Mesa 10.4.1 and Mesa git master, both are effected by this issue.  Looks
like it was introduced in the 10.4 branch.  Attempting to bisect.