drm/i915: Apply some ocd for IMR vs. IER order during irq enable

When disabling interrupts we do the writes in this order:
IMR,IER,IIR,IIR. But when enabling interrupts we don't do use the
mirrored order, and instead do IIR,IIR,IMR,IER.

I like consistency unless there's a good reason against it, which I
can't think of here, so change the enable order to IIR,IIR,IER,IMR.

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>