drm/i915: Use vblank_disable_immediate on gen2

The vblank timestamp->counter guesstimator seems to be
working sufficiently well, so there's no reason not to
disable vblank interrupts ASAP even on gen2.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190322180804.3300-2-ville.syrjala@linux.intel.com
Acked-by: Chris Wilson <chris@chris-wilson.co.uk>