drm/i915: rip out old HWSTAM missed irq WA for vlv

This got copy-pasted from an older version. The newer kinds of
workarounds don't need this anymore.

Shame on me for not noticing when picking up the vlv irq patch.

Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>