i915: Disable MSI on GM965 (errata says it doesn't work)

Current Intel errata for the GM965 says that using MSI may cause interrupts
to be delayed or lost. The only workaround offered is to not use it.

Signed-off-by: Keith Packard <keithp@keithp.com>
Signed-off-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Dave Airlie <airlied@redhat.com>