drm/st7586: Use framebuffer dma-buf helpers

Replace dma_buf_begin_cpu_access() with drm_gem_fb_begin_cpu_access();
same for _end_cpu_access(). Remove some boiler-plate code. No functional
changes.
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Noralf Trønnes <noralf@tronnes.org>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: David Lechner <david@lechnology.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210716140801.1215-8-tzimmermann@suse.de

drivers/gpu/drm/tiny/st7586.c

@@ -6,7 +6,6 @@
  */
 
 #include <linux/delay.h>
-#include <linux/dma-buf.h>
 #include <linux/gpio/consumer.h>
 #include <linux/module.h>
 #include <linux/property.h>
@@ -21,6 +20,7 @@
 #include <drm/drm_format_helper.h>
 #include <drm/drm_gem_atomic_helper.h>
 #include <drm/drm_gem_cma_helper.h>
+#include <drm/drm_gem_framebuffer_helper.h>
 #include <drm/drm_managed.h>
 #include <drm/drm_mipi_dbi.h>
 #include <drm/drm_rect.h>
@@ -92,24 +92,18 @@ static int st7586_buf_copy(void *dst, struct drm_framebuffer *fb,
 			   struct drm_rect *clip)
 {
 	struct drm_gem_cma_object *cma_obj = drm_fb_cma_get_gem_obj(fb, 0);
-	struct dma_buf_attachment *import_attach = cma_obj->base.import_attach;
 	void *src = cma_obj->vaddr;
 	int ret = 0;
 
-	if (import_attach) {
-		ret = dma_buf_begin_cpu_access(import_attach->dmabuf,
-					       DMA_FROM_DEVICE);
-		if (ret)
-			return ret;
-	}
+	ret = drm_gem_fb_begin_cpu_access(fb, DMA_FROM_DEVICE);
+	if (ret)
+		return ret;
 
 	st7586_xrgb8888_to_gray332(dst, src, fb, clip);
 
-	if (import_attach)
-		ret = dma_buf_end_cpu_access(import_attach->dmabuf,
-					     DMA_FROM_DEVICE);
+	drm_gem_fb_end_cpu_access(fb, DMA_FROM_DEVICE);
 
-	return ret;
+	return 0;
 }
 
 static void st7586_fb_dirty(struct drm_framebuffer *fb, struct drm_rect *rect)