drm/i915: extract gt interrupt handler

vlv, ivb and snb all share the gen6+ gt irq handling. 3 copies of the
same stuff is a bit much, so extract it into a little helper.

Now ilk has a different gt irq handling than snb, but shares the same
irq handler (due to the similar display block). So also extract the
ilk gt irq handling to clearly separate these two things.

Nice side effect of this is that we can complete Ben Widawsky's gen6+
irq bit #define cleanup and call the render irq also with the GEN6
alias. Beforehand that code was shared with ilk, and neither option
really made much sense.

As a bonus this enables the error interrupt handling lifted from the
vlv code on snb and ivb, too.

Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Antagonized-by: Ben Widawsky <ben@bwidawsk.net>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>