drm/i915/skl: split skl_compute_ddb function

This patch splits skl_compute_wm/ddb functions into two parts.
One adds all affected pipes after the commit to atomic_state structure
and second part does compute the DDB.

v2: Added reviewed by tag from Shashank Sharma

v3: Added reviewed by from Juha-Pekka Heikkila

v4: Rebased the series

v5: Fixed checkpatch error. Changed *changed = true
to (*changed) = true;

Reviewed-by: Juha-Pekka Heikkila <juhapekka.heikkila@gmail.com>
Reviewed-by: Shashank Sharma <shashank.sharma@intel.com>
Signed-off-by: Mahesh Kumar <mahesh1.kumar@intel.com>
Signed-off-by: Vidya Srinivas <vidya.srinivas@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1523245273-30264-10-git-send-email-vidya.srinivas@intel.com