i915: make vbl interrupts work properly on i965g/gm hw.

This code is ported from the DRM git tree and allows the vblank interrupts
to function on the i965 hw. It also requires a change in Mesa's 965 driver
to actually use them.

[ Without this patch, my 965GM drops vblank interrupts  - Jesse ]

Signed-off-by: Dave Airlie <airlied@linux.ie>
Acked-by: Jesse Barnes <jesse.barnes@intel.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>