drm/i915: Merge duplicate gen4 and vlv/chv enable vblank callbacks

gen4/vlv/chv all use the same bits in pipestat to enable the vblank
interrupt, so they can share the same callbacks to enable/disable.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161007194953.15616-1-chris@chris-wilson.co.uk
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>