drm/i915: Filter gmch fifo underruns in the shared handler

This simplifies the code in the vlv irq handler. Also this now
means that we correctly filter underruns on gen2-4.

And as the real upshot I need to document one less function for
the fifo underrun code.

v2: Shorten one long line.

Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>