drm/i915: Use vlv display irq setup code for chv

Throw away the hand rolled display irq setup code on chv, and instead
just call vlv_display_irq_postinstall() and vlv_display_irq_uninstall().

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>