drm/i915: Break out common code from gen8_gt_irq_handler

drm/i915: Break out common code from gen8_gt_irq_handler

Break out common code from gen8_gt_irq_handler and put it in to
an always inlined function. gcc optimises out the shift at compile
time. (Thomas Daniel/Daniel Vetter/Chris Wilson)

Issue: VIZ-4277
Signed-off-by: Nick Hoath <nicholas.hoath@intel.com>
Cc: Thomas Daniel <thomas.daniel@intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1445333036-22164-3-git-send-email-nicholas.hoath@intel.com
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>