drm/i915/skl+: refactor WM calculation for NV12

Current code calculates DDB for planar formats in such a way that we
store DDB of plane-0 in plane 1 & vice-versa.
In order to make this clean this patch refactors WM/DDB calculation for
NV12 planar formats.

v2: Addressed review comments by Maarten

v3: Rebased and addressed review comments by Maarten

v4: Fixed a compilation issue of string replacement is_nv12 to
is_planar

v5: Added reviewed by from Juha-Pekka Heikkila

v6: 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: Vidya Srinivas <vidya.srinivas@intel.com>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1523245273-30264-3-git-send-email-vidya.srinivas@intel.com