drm/i915/panel: setup pwm backlight based on connector type

Use the connector type instead of VBT directly to decide which backlight
mechanism to use on VLV/CHV. (Indirectly, this is the same thing, but
hides the VBT use.)

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1458125015-7931-5-git-send-email-jani.nikula@intel.com