Min brightness value from vbt was missing for BXT platform.
This setting have to refer backlight ic spec to restrict
min backlight output. Without this restriction, driver would
allow to configure lower brightness value and violate
backlight ic requirement.

Fixes: 0fb890c0 ("drm/i915/bxt: BLC implementation")
Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Cooper Chiou <cooper.chiou@intel.com>
Cc: Gary C Wang <gary.c.wang@intel.com>
Signed-off-by: NShawn Lee <shawn.c.lee@intel.com>
Signed-off-by: NJani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1505187390-7039-1-git-send-email-shawn.c.lee@intel.com
(cherry picked from commit c3881128)
Signed-off-by: NRodrigo Vivi <rodrigo.vivi@intel.com>