drm/i915/skl+: rename skl_wm_values struct to skl_ddb_values

skl_wm_values struct contains values of pipe/plane DDB only.
so rename it for better readability of code. Similarly
skl_copy_wm_for_pipe copies DDB values.

s/skl_wm_values/skl_ddb_values
s/skl_copy_wm_for_pipe/skl_copy_ddb_for_pipe

Changes since V1:
 - also change name of skl_copy_wm_for_pipe

v2: Added reviewed by from Juha-Pekka Heikkila

v3: Rebased the series

Reviewed-by: Juha-Pekka Heikkila <juhapekka.heikkila@gmail.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Mahesh Kumar <mahesh1.kumar@intel.com>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1523245273-30264-2-git-send-email-vidya.srinivas@intel.com