drm/i915: rip out the HWSTAM missed irq workaround

With the new ducttape of much finer quality, this seems to be no
longer necessary.

Tested on my ivb and snb machine with the usual suspects of testcases.

(v2 by keithp -- limited change to IVB only for now)

Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Keith Packard <keithp@keithp.com>