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
diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
index c1b89e9..28fd200 100644
--- a/drivers/gpu/drm/i915/i915_drv.h
+++ b/drivers/gpu/drm/i915/i915_drv.h
@@ -1182,8 +1182,9 @@ static inline bool skl_ddb_entry_equal(const struct skl_ddb_entry *e1,
 }
 
 struct skl_ddb_allocation {
-	struct skl_ddb_entry plane[I915_MAX_PIPES][I915_MAX_PLANES]; /* packed/uv */
-	struct skl_ddb_entry y_plane[I915_MAX_PIPES][I915_MAX_PLANES];
+	/* packed/y */
+	struct skl_ddb_entry plane[I915_MAX_PIPES][I915_MAX_PLANES];
+	struct skl_ddb_entry uv_plane[I915_MAX_PIPES][I915_MAX_PLANES];
 };
 
 struct skl_ddb_values {