drm: rcar-du: Use __drm_atomic_helper_plane_reset instead of copying the logic

A new helper function(__drm_atomic_helper_plane_reset) has been added
for linking a plane with its state and resetting the core
properties(alpha, rotation, etc.) to their default values.
Use that instead of duplicating the logic.

__drm_atomic_helper_plane_reset initializes the alpha property to its
max value, which is defined by the drm core as DRM_BLEND_ALPHA_OPAQUE,
so nothing changes regarding the alpha value.

Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Alexandru Gheorghe <alexandru-cosmin.gheorghe@arm.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180804161530.12275-8-alexandru-cosmin.gheorghe@arm.com

drivers/gpu/drm/rcar-du/rcar_du_plane.c

@@ -690,14 +690,12 @@ static void rcar_du_plane_reset(struct drm_plane *plane)
 	if (state == NULL)
 		return;
 
+	__drm_atomic_helper_plane_reset(plane, &state->state);
+
 	state->hwindex = -1;
 	state->source = RCAR_DU_PLANE_MEMORY;
 	state->colorkey = RCAR_DU_COLORKEY_NONE;
 	state->state.zpos = plane->type == DRM_PLANE_TYPE_PRIMARY ? 0 : 1;
-
-	plane->state = &state->state;
-	plane->state->alpha = DRM_BLEND_ALPHA_OPAQUE;
-	plane->state->plane = plane;
 }
 
 static int rcar_du_plane_atomic_set_property(struct drm_plane *plane,

drivers/gpu/drm/rcar-du/rcar_du_vsp.c

@@ -346,11 +346,8 @@ static void rcar_du_vsp_plane_reset(struct drm_plane *plane)
 	if (state == NULL)
 		return;
 
-	state->state.alpha = DRM_BLEND_ALPHA_OPAQUE;
+	__drm_atomic_helper_plane_reset(plane, &state->state);
 	state->state.zpos = plane->type == DRM_PLANE_TYPE_PRIMARY ? 0 : 1;
-
-	plane->state = &state->state;
-	plane->state->plane = plane;
 }
 
 static const struct drm_plane_funcs rcar_du_vsp_plane_funcs = {