drm/i915: Drop useless VLV_IIR writes from vlv_display_irq_postinstall()

The extra VLV_IIR writes at the end of vlv_display_irq_postinstall()
serve no purpose. Remove them.

The VLV_IMR/IER/IIR setup at the start of the function also seems a bit
pointless since it doesn't unmask/enable anything. But leave it be for
now.

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>