drm/i915: don't trigger ironlake vblank interrupt at irq install

Zhenyu noticed that the ironlake vblank enabling patch has one
issue that it will trigger vblank starting from irq postinstall,
this isn't necessary. This patch addresses this issue by only
adding the vblank into DEIER but mask them in DEIMR, so that it
won't trigger vblank interrupt at irq install.

Signed-off-by: Li Peng <peng.li@intel.com>
Acked-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Eric Anholt <eric@anholt.net>