drm/i915: Disable the GM965 MSI errata workaround.

Since applying the fix suggested by the errata (disabling MSI), we've had
issues with interrupts being stuck on despite IIR being 0 on GM965 hardware.
Most reporters of the issue have confirmed that turning MSI back on fixes
things, and given the difficulties experienced in getting reliable MSI working
on Linux, it's believable that the errata was about software issues and not
actual hardware issues.

Signed-off-by: Dave Airlie <airlied@redhat.com>