drm/i915/gen8: Factor out display interrupt handling

Tidy quite long interrupt service routine by factoring out
the display part.

This simplifies the exit path a little bit, makes the code
a bit more readable, and potentialy makes code reuse in the
future easier.

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/1452614647-13973-2-git-send-email-tvrtko.ursulin@linux.intel.com