drm/i915: Track which port is using which pipe's power sequencer

VLV/CHV have a per-pipe panel power sequencer which locks onto the
port once used. We need to keep track wich power sequencers are
locked to which ports.

v2: remove spurious whitespace change, rebase due to backlight changes (Imre)

Reviewed-by: Antti Koskipaa <antti.koskipaa@linux.intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
[danvet: Break some really long lines to appease checkpatch a bit.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>