drm/i915: Make valleyview_display_irqs_(un)install() work for chv

Genralize valleyview_display_irqs_install() and
valleyview_display_irqs_uninstall() enough so that they work on chv.
The only difference to vlv here being the third pipe that chv brings.

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>