drm/i915: Drop the extra GEN8_PCU_IIR posting read from cherryview_irq_preinstall()

Looks like a leftover POSTING_READ(GEN8_PCU_IIR) in
cherryview_irq_preinstall() from some earlier age. GEN5_IRQ_RESET()
already does the posting read so this changes nothing, so kill it.

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>