drm/i915/gt: Move intel_breadcrumbs_arm_irq earlier

Move the __intel_breadcrumbs_arm_irq earlier, next to the disarm_irq, so
that we can make use of it in the following patch.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200801160225.6814-1-chris@chris-wilson.co.uk