drm/i915/icl: Enable DDI Buffer

This patch enables DDI buffer by writing to DDI_BUF_CTL
register and wait for DDI status to be *not idle* for a
port.

v2: Rebase
v3: Remove step hard coding comments (Jani N)

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/1537095223-5184-4-git-send-email-madhav.chauhan@intel.com