drm/i915: also use GEN5_IRQ_INIT with south display interrupts

This interrupt gets initialized with a different IER value, so it was
not using the macro. The problem is that we plan to modify the macro
to make it do additional things, and we want the SDE interrupts
updated too. So let's make sure we call the macro, then, after it, we
do the necessary SDE-specific changes.

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>