drm/i915: Refactor vlv_display_irq_uninstall()

Pull the vlv display irq uninstall code into a separate function, for
eventual sharing 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>