drm/i915: extract intel_dp_init_panel_power_sequencer

That thing has grown way too big already.

Also move around a comment to the right spot.

Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Tested-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>