drm/i915/icl: Enable both DBuf slices during init

ICL has 2 slices of DBuf, enable both the slices during display init.

Ideally we should only enable the second slice when needed in order to
save power, but while we're not there yet, adopt the simpler solution
to keep us bug-free.

v2 (from Paulo):
  - Add the TODO comment.
  - Reorganize where things are defined.
  - Fix indentation.
  - Remove unnecessary POSTING_READ() calls.
  - Improve the commit message.

Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Mahesh Kumar <mahesh1.kumar@intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180205154046.11485-5-paulo.r.zanoni@intel.com