drm/i915/bxt: set min brightness from VBT

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: Shawn Lee <shawn.c.lee@intel.com>
Signed-off-by: Jani 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: Rodrigo Vivi <rodrigo.vivi@intel.com>