drm/ast: Replace struct_drm_device.dev_private with to_ast_private()

The ast code still references dev_private in several place when looking
up the ast device structure. Convert the remaining locations to use
to_ast_private().
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200730135206.30239-5-tzimmermann@suse.de

drivers/gpu/drm/ast/ast_cursor.c

@@ -47,7 +47,7 @@ static void ast_cursor_fini(struct ast_private *ast)
 
 static void ast_cursor_release(struct drm_device *dev, void *ptr)
 {
-	struct ast_private *ast = dev->dev_private;
+	struct ast_private *ast = to_ast_private(dev);
 
 	ast_cursor_fini(ast);
 }

drivers/gpu/drm/ast/ast_mode.c

@@ -663,7 +663,7 @@ ast_cursor_plane_helper_atomic_update(struct drm_plane *plane,
 {
 	struct drm_plane_state *state = plane->state;
 	struct drm_framebuffer *fb = state->fb;
-	struct ast_private *ast = plane->dev->dev_private;
+	struct ast_private *ast = to_ast_private(plane->dev);
 	unsigned int offset_x, offset_y;
 
 	offset_x = AST_MAX_HWC_WIDTH - fb->width;