drm/i915: Create an ivybridge_irq_preinstall

Just duplicates ironlake_irq_preinstall for now.

v2: Add new PCH_NOP check (Damien)
Add SDEIMR comment (Damien)

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
[danvet: Update now outdated comment.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>