drm/i915/vlv: use per-pipe backlight controls v2

With the connector and pipe passed around, we can now set the backlight
on the right pipe on VLV/BYT.

v2: drop combination mode check for VLV (Jani)
    add save/restore code for VLV backlight regs (Jani)
    check for existing modulation freq when initializing backlight regs (Jani)

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=67245


Tested-by: Joe Konno <joe.konno@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>