drm/i915: factor out intel_set_cpu_fifo_underrun_reporting_nolock

Needed by the next patch, wanting to set the underrun reporting as part
of a bigger dev_priv->irq_lock'ed sequence.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
[danvet: Use more customary __ prefix instead of _nolock postfix.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>