drm/i915: wire up gmbus irq handler

Only enables the interrupt and puts a irq handler into place, doesn't
do anything yet.

Unfortunately there's no gmbus interrupt support for gen2/3 (safe for
pnv, but there the irq is marked as "Test mode").

v2: Wire up the irq handler for vlv and gen4 properly.

v3: i915_enable_pipestat expects the mask bit, not the status bits ... and
for added hilarity those are rather inconsistently named.

Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>