drm/fb-helper: Call fb_sync in I/O functions

Call struct fb_ops.fb_sync in drm_fbdev_{read,write}() to mimic the
behavior of fbdev. Fbdev implementations of fb_read and fb_write in
struct fb_ops invoke fb_sync to synchronize with outstanding operations
before I/O. Doing the same in DRM implementations will allow us to use
them throughout DRM drivers.

Signed-off-by: Thomas Zimmermann <[e-mail]>
Reviewed-by: Javier Martinez Canillas <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/drm_fb_helper.c

···
 	if (total_size - count < pos)
 		count = total_size - pos;
 
+	if (info->fbops->fb_sync)
+		info->fbops->fb_sync(info);
+
 	if (drm_fbdev_use_iomem(info))
 		ret = fb_read_screen_base(info, buf, count, pos);
 	else
···
 			err = -ENOSPC;
 		count = total_size - pos;
 	}
+
+	if (info->fbops->fb_sync)
+		info->fbops->fb_sync(info);
 
 	/*
 	 * Copy to framebuffer even if we already logged an error. Emulates