drm/udl: Use damage iterator

Use a damage iterator to process damage areas individually. Merging
damage areas can result in large updates of unchanged framebuffer
regions. As USB is rather slow, it's better to process damage areas
individually and hence minimize USB-transfered data.

As part of the change, move drm_gem_fb_{begin,end}_cpu_access() into
the plane's atomic_update helper. To avoid overhead and intermediate
writers, we want to synchronize buffers and reserve access only once
before copying damage areas of the framebuffer.

v2:
	* clarify commit message (Javier)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221006095355.23579-11-tzimmermann@suse.de

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