drm/i915/icl: initialize MBus during display init

This patch initializes MBus during display initialization.

Changes since V2 (from Paulo):
 - Don't forget to remove the WARN_ON(1) call.
Changes since V1:
 - Rebase to use function like Macros

Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: James Ausmus <james.ausmus@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/20180205154046.11485-6-paulo.r.zanoni@intel.com