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 <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221103151446.2638-17-tzimmermann@suse.de