drm/i915: Refactor vlv_display_irq_reset()

Pull the vlv display irq reset code to a new functions. The aim is to
share the code with chv.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>