drm/i915: refactor ilk display interrupt handling

- Use a for_each_loop and add the corresponding #defines.
- Drop the _ILK postfix on the existing DE_PIPE_VBLANK macro for
  consistency with everything else.
- Also use macros (and add the missing one for plane flips) for the
  ivb display interrupt handler.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
[danvet: Drop the useless parens that Ville spotted.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>