drm/i915: kill Ivybridge vblank irq vfuncs

The IVB funtions are exactly the same as the ILK ones, with the
exception of the bit register. So add IVB/HSW support to
ironlake_enable_vblank and ironlake_disable_vblank, then kill the
ivybridge functions.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>