drm/i915: Compact logical ring interrupt initialization

Identically to vfuncs interrupt mask initialization can also be
compacted for more readable code.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1452619956-27014-2-git-send-email-tvrtko.ursulin@linux.intel.com