drm/i915: Adding dbuf support for skl nv12 format.

Skylake nv12 format requires dbuf (aka. ddb) calculations
and programming for each of y and uv sub-planes. Made minor
changes to reuse current dbuf calculations and programming
for uv plane. i.e., with this change, existing computation
is used for either packed format or uv portion of nv12
depending on incoming format. Added new code for dbuf
computation and programming for y plane.

This patch is a pre-requisite for adding NV12 format support.
Actual nv12 support is coming in later patches.

Signed-off-by: Chandra Konduru <chandra.konduru@intel.com>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
index acfa4fc..6b3142a 100644
--- a/drivers/gpu/drm/i915/i915_drv.h
+++ b/drivers/gpu/drm/i915/i915_drv.h
@@ -1468,7 +1468,8 @@
 
 struct skl_ddb_allocation {
 	struct skl_ddb_entry pipe[I915_MAX_PIPES];
-	struct skl_ddb_entry plane[I915_MAX_PIPES][I915_MAX_PLANES];
+	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]; /* y-plane */
 	struct skl_ddb_entry cursor[I915_MAX_PIPES];
 };