drm/i915: Extract PIPESTAT irq handling into separate functions

Extract the gen2-4 PIPESTAT irq handling into separate functions just
like we already do on VLV/CHV.

We can share valleyview_pipestat_irq_ack() on all gmch platforms to
actually read and clear the PIPESTAT status bits, so let's rename
it to i9xx_pipestat_irq_ack().

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170818183705.27850-11-ville.syrjala@linux.intel.com