drm/i915/cnp: set min brightness from VBT

Min brightness value from vbt was missing for CNP 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: 4c9f7086

 ("drm/i915/cnp: Backlight support for CNP.")
Cc: Jani Nikula <jani.nikula@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/1505279961-16140-1-git-send-email-shawn.c.lee@intel.com