drm/i915: apply CS reg readback trick against missed IRQ on snb

Ben Widawsky reported missed IRQ issues and this patch here helps.

We have one other missed IRQ report still left on snb, reported by QA:

https://bugs.freedesktop.org/show_bug.cgi?id=46145



This is _not_ a regression due to the forcewake voodoo though, it
started showing up before that was applied and has been on-and-off for
the past few weeks. According to QA this patch does not help. But the
missed IRQ is always from the blt ring (despite running piglit, so
also render activity expected), so I'm hopefully that this is an issue
with the blt ring itself.

Tested-by: Ben Widawsky <ben@bwidawsk.net>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>