drm/i915/gen8: Tidy display interrupt processing

One bugfix and a few tidy-ups:

 * Pipe fault logging was broken on Gen9+.
 * Removed some unnecessary local variables.
 * Removed unnecessary initializers.
 * Decreased pipe iir block indentation level.
 * Grouped variable initialization close to use sites.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Chris Wilson <chris@cris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1452614647-13973-1-git-send-email-tvrtko.ursulin@linux.intel.com