drm/i915/icl: track dbuf slice-2 status

This patch adds support to start tracking status of DBUF slices.
This is foundation to introduce support for enabling/disabling second
DBUF slice dynamically for ICL.

Changes Since V1:
 - use kernel type u8 over uint8_t

Signed-off-by: Mahesh Kumar <mahesh1.kumar@intel.com>
Reviewed-by: James Ausmus <james.ausmus@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180426142517.16643-2-mahesh1.kumar@intel.com