drm/i915: Introduce bdw_{update,enable,disable}_pipe_irq()

Pull the BDW+ DE pipe interrupt mask frobbing into a central place,
like we have for other platforms.

v2: Fix the kerneldoc (Daniel)

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1448294777-13722-4-git-send-email-ville.syrjala@linux.intel.com


Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>