drm/i915: kill ivybridge_irq_postinstall

It was very similar to ironlake_irq_postinstall, so IMHO merging both
functions results in a code that is easier to maintain.

With this change, all the irq handler vfuncs between ironlake and
ivybridge are now unified.

v2: Add "(" and ")" to make at least one vim user much happier (Chris)

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>