drm/i915: add GEN5_IRQ_INIT macro

The goal is to reuse the GEN8 macros, but a few changes are needed, so
let's make things easier to review.

I could also use these macros on older code, but since I plan to
change how the interrupts are initialized, we'll risk breaking the
older code in the next commits, so I'll leave this out for now.

v2: - Rebase.

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