drm/i915/icl: Turn ON panel backlight

This patch enables backlight of DSI panel by using VBT
BACKLIGHT_ON sequence and panel specific functions.

Signed-off-by: Madhav Chauhan <madhav.chauhan@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/05987eea297689af0b9defcf745a921f76f9011d.1540900289.git.jani.nikula@intel.com