drm/i915: vlv: keep first level vblank IRQs masked

This is a left-over from

commit b7e634cc
Author: Imre Deak <imre.deak@intel.com>
Date:   Tue Feb 4 21:35:45 2014 +0200

drm/i915: vlv: don't unmask IIR[DISPLAY_PIPE_A/B_VBLANK] interrupt

where we stopped unmasking the vblank IRQs, but left them enabled in the
IER register. Disable them in IER too.

v2:
- remove comment becoming stale after this change (Ville)

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>