drm/i915/skl+: Watermark calculation cleanup

This patch cleanup/reorganises the watermark calculation functions.
This patch make use of already available macro
"drm_atomic_crtc_state_for_each_plane_state" to walk through
plane_state list instead of calculating plane_state in function itself.

This restructuring will help later patch for new DDB allocation
algorithm to do only algo related changes.

Changes from V1:
 - split the patch in two parts as per Matt's comment

Signed-off-by: Mahesh Kumar <mahesh1.kumar@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170517115831.13830-9-mahesh1.kumar@intel.com