drm/i915: Simplify interrupt processing for IvyBridge

We can take advantage that the PCH_IIR is a subordinate register to
reduce one of the required IIR reads, and that we only need to clear
interrupts handled to reduce the writes. And by simply tidying the code
we can reduce the line count and hopefully make it more readable.

v2: Split out the bugfix from the refactoring.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>