drm/i915: Optimize pipe irq handling on bdw

We have a per-pipe bit in the master irq control register, so use it.
This allows us to drop the masks for aggregate interrupt bits and be a
bit more explicit in the code. It also removes one indentation level.

Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>