i965: Apply a workaround for the Ironlake "vertex flashing".

i965: Apply a workaround for the Ironlake "vertex flashing".

This is an awful hack and will hurt performance on Ironlake, but we're
at a loss as to what's going wrong otherwise.  This is the only common
variable we've found that avoids the problem on 4 applications
(CelShading, gnome-shell, Pill Popper, and my GLSL demo), while other
variables we've tried appear to only be confounding.  Neither the
specifications nor the hardware team have been able to provide any
enlightenment, despite much searching.

https://bugs.freedesktop.org/show_bug.cgi?id=29172
Tested by:	Chris Lord <chris@linux.intel.com> (Pill Popper)
Tested by:	Ryan Lortie <desrt@desrt.ca> (gnome-shell)