drm/i915: add GEN5_IRQ_INIT

And the equivalent GEN8_IRQ_INIT_NDX macro. These macros are for the
postinstall functions. The next patch will improve this macro.

v2: - Adjust to the new POSTING_READ scheme (Ben).

Reviewed-by: Ben Widawsky <ben@bwidawsk.net> (v1)
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>