drm/i915: Use a consistent order between IIR, IER, IMR writes on vlv/chv

Follow the same ordering rules for the IIR,IER,IMR writes on vlv/chv
that we do on other gen5+ platforms.

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>