drm/i915: reuse Ivy Bridge interrupts code for Haswell

Haswell interrupts are mostly similar with Ivy Bridge, so we share same
routines with it.

This patch also simplifies the vblank counter handling for all the Gen5+
architectures.

Signed-off-by: Eugeni Dodonov <eugeni.dodonov@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>